全文获取类型
收费全文 | 3222篇 |
免费 | 311篇 |
国内免费 | 253篇 |
专业分类
3786篇 |
出版年
2024年 | 5篇 |
2023年 | 64篇 |
2022年 | 63篇 |
2021年 | 88篇 |
2020年 | 106篇 |
2019年 | 136篇 |
2018年 | 114篇 |
2017年 | 134篇 |
2016年 | 130篇 |
2015年 | 144篇 |
2014年 | 165篇 |
2013年 | 254篇 |
2012年 | 130篇 |
2011年 | 153篇 |
2010年 | 117篇 |
2009年 | 158篇 |
2008年 | 184篇 |
2007年 | 159篇 |
2006年 | 173篇 |
2005年 | 127篇 |
2004年 | 125篇 |
2003年 | 105篇 |
2002年 | 100篇 |
2001年 | 93篇 |
2000年 | 84篇 |
1999年 | 72篇 |
1998年 | 57篇 |
1997年 | 49篇 |
1996年 | 37篇 |
1995年 | 42篇 |
1994年 | 42篇 |
1993年 | 34篇 |
1992年 | 38篇 |
1991年 | 24篇 |
1990年 | 24篇 |
1989年 | 26篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 15篇 |
1985年 | 29篇 |
1984年 | 27篇 |
1983年 | 14篇 |
1982年 | 17篇 |
1981年 | 19篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1972年 | 6篇 |
排序方式: 共有3786条查询结果,搜索用时 15 毫秒
91.
单核苷酸多态性(single nucleotide polymorphism,SNPs),即在基因组水平上由单个核苷酸的变异而引起的DNA序列多态性变化,具体是指在DNA序列中的单个碱基的变异,其是人类基因组变异种最常见的一种。SNP研究最主要的目的就是对人类表型变异遗传学的理解,尤其是关于人类遗传疾病的研究。而非同义单核苷酸多态性(nsSNPs)是SNPs中的一种,主要是指处于编码区会引起翻译后对应氨基酸序列变化的单核苷酸突变。因为nsSNPs可能会对蛋白质的功能造成影响,被认为是造成人类遗传病的主要原因。因此将与疾病相关的nsSNPs从中性的nsSNPs中区分出来是很重要的。本文根据国内外与疾病相关nsSNPs预测的研究,分析了预测中所涉及到的特征属性,总结了对这些特征进行优化的特征选择方法,并概述了在预测过程中使用的各种分类器。 相似文献
92.
西藏南部岗巴地区白垩纪中期钙质超微化石带和Cenomanian-Turonian界线 总被引:6,自引:0,他引:6
我国西藏南部海相白垩系含有较丰富的钙质超微化石。文中着重研究岗巴地区两个剖面 (即剖面A ,B)Albian Santonian钙质超微化石的分布。根据标志种的存在 ,识别出 5个初现面事件 ,相应地建立 6个钙质超微化石带 ,自下至上是Prediscosphaeracretacea带 ,Eiffellithusturriseiffeli带 ,Lithraphiditesacutum带 ,Gartneragoobliquum带 ,Quadrumgartneri带 ,Lucianorhabduscayeuxii带。同时 ,通过洲际对比 ,建议以G .obliquum初现面作为划分本区Cenomanian和Turonian界线的标志。此外 ,Q .gartneri带和I .cayeuxii带之间缺失多个化石带 ,据此推测Turonian至Santonian期间本区可能存在沉积间断。 相似文献
93.
Long-term Perspectives on Lagged Ecosystem Responses to Climate Change: Permafrost in Boreal Peatlands and the Grassland/Woodland Boundary 总被引:3,自引:1,他引:3
Changes in climate could have far-reaching consequences for ecosystems sensitive to changes in temperature and precipitation,
such as boreal permafrost peatlands and grassland/woodland boundaries. The long-term data from our studies in these ecosystems
suggest that transient responses of permafrost and vegetation to climate change may be difficult to predict due to lags and
positive feedbacks related to vegetation and disturbance. Boreal permafrost peatlands comprise an ecosystem with strong local
controls on microclimate that influence the formation and thaw of permafrost. These local controls may preserve permafrost
during the transient stages of climate warming, producing lagged responses. The prairie–forest border region of the northern
Great Plains has experienced frequent change and has complex dynamics involving transitions in the grassland composition of
prairie and in the degree of woodiness in bordering forests. Fire frequency interacts with fuel loading and tree recruitment
in ways that affect the timing and direction of change. Lags and thresholds could lead to sudden large responses to future
climate change that are not readily apparent from current vegetation. The creation of adequate models to characterize transient
ecosystem changes will require an understanding of the linkages among processes operating at the scale of 10s of meters and
over long time periods.
Received 14 December 1999; accepted 7 July 2000. 相似文献
94.
95.
Piyumika S. Suriyampola Perri K. Eason 《Ethology : formerly Zeitschrift fur Tierpsychologie》2015,121(8):785-792
Defense of a territory is a primary factor determining the fitness of territorial individuals. Thus, territorial animals use a variety of means to lower the cost of territorial defense. Although landmarks are known to lower the defense costs of territories, little is known about how they could affect fundamental properties of territories, which could ultimately affect the demography, population regulation, individual behavior, and spatial ecology of a territorial species. This field study examined the effects of natural landmarks on territory size, shape, and defensive costs in Amatitlania siquia, a little‐studied member of the convict cichlid group native to Costa Rica and Nicaragua. We investigated the territorial behavior of 103 breeding pairs in a caldera lake in Nicaragua. During 10‐min focal observations, we recorded the locations at which residents chased intruders and then used these locations to outline the territory boundaries. Landmarks affected both size and shape of A. siquia territories. Territory size decreased as the percentage of the boundary at landmarks increased. Territories with <20% of their boundaries at landmarks were approximately circular in shape, while territories with 20–60% of their boundaries at landmarks were on average more elongated. These variations in territory size and shape significantly affected defense costs, with higher numbers of chases per square meter in small and elongated territories. These effects of landmarks on territory size and shape and defense costs are useful to understand the widely seen size and shape variations in naturally occurring territories. 相似文献
96.
《Molecular & cellular proteomics : MCP》2019,18(12):2516-2523
Highlights
- •Open source software for comprehensive HDX-MS data analysis.
- •Automatic back-exchange correction options.
- •Rigorous statistical analysis of the significance of uptake differences.
- •High quality visualization tools.
97.
The late-third-instar labial disc is comprised of two disc-proper cell layers, one representing mainly the ventral half of the anterior compartment (L-layer) and the other, the dorsal half of the anterior compartment and most, if not all, of the posterior compartment (M-layer). In the L-layer, Distal-less represses homothorax whereas no Distal-less-dependent homothorax repression occurs in the M-layer where Distal-less is coexpressed with homothorax. In wild-type labial discs, clawless, one of the two homeobox genes expressed in distal cells receiving maximum (Decapentaplegic+Wingless) signaling activity in leg and antennal discs, is specifically repressed by proboscipedia. A fate map, inferred from data on basic patterning gene expression in larval and pupal stages and mutant phenotypes, indicates the inner surface of the labial palpus, which includes the pseudotracheal region, to be a derivative of the distal portion of the M-layer expressing wingless, patched, Distal-less and homothorax. The outer surface of the labial palpus with more than 30 taste bristles derives from an L-layer area consisting of dorsal portions of the anterior and posterior compartments, each expressing Distal-less. Our analysis also indicates that, in adults and pupae, the anterior-posterior boundary, dividing roughly equally the outer surface of the distiproboscis, runs along the outer circumference of the inner surface of distiproboscis. 相似文献
98.
Michael?WangEmail author Hanjie?Lee John?Molburg 《The International Journal of Life Cycle Assessment》2004,9(1):34-44
Aim, Scope, and Background Studies to evaluate the energy and emission impacts of vehicle/fuel systems have to address allocation of the energy use and
emissions associated with petroleum refineries to various petroleum products because refineries produce multiple products.
The allocation is needed in evaluating energy and emission effects of individual transportation fuels. Allocation methods
used so far for petroleum-based fuels (e.g., gasoline, diesel, and liquefied petroleum gas [LPG]) are based primarily on mass,
energy content, or market value shares of individual fuels from a given refinery. The aggregate approach at the refinery level
is unable to account for the energy use and emission differences associated with producing individual fuels at the next sub-level:
individual refining processes within a refinery. The approach ignores the fact that different refinery products go through
different processes within a refinery. Allocation at the subprocess level (i.e., the refining process level) instead of at
the aggregate process level (i.e., the refinery level) is advocated by the International Standard Organization. In this study,
we seek a means of allocating total refinery energy use among various refinery products at the level of individual refinery
processes.
Main Features We present a petroleum refinery-process-based approach to allocating energy use in a petroleum refinery to petroleum refinery
products according to mass, energy content, and market value share of final and intermediate petroleum products as they flow
through refining processes within a refinery. The approach is based on energy and mass balance among refining processes within
a petroleum refinery. By using published energy and mass balance data for a simplified U.S. refinery, we developed a methodology
and used it to allocate total energy use within a refinery to various petroleum products. The approach accounts for energy
use during individual refining processes by tracking product stream mass and energy use within a refinery. The energy use
associated with an individual refining process is then distributed to product streams by using the mass, energy content, or
market value share of each product stream as the weighting factors.
Results The results from this study reveal that product-specific energy use based on the refinery process-level allocation differs
considerably from that based on the refinery-level allocation. We calculated well-to-pump total energy use and greenhouse
gas (GHG) emissions for gasoline, diesel, LPG, and naphtha with the refinery process-based allocation approach. For gasoline,
the efficiency estimated from the refinery-level allocation underestimates gasoline energy use, relative to the process-level
based gasoline efficiency. For diesel fuel, the well-to-pump energy use for the process-level allocations with the mass- and
energy-content-based weighting factors is smaller than that predicted with the refinery-level allocations. However, the process-level
allocation with the market-value-based weighting factors has results very close to those obtained by using the refinery-level
allocations. For LPG, the refinery-level allocation significantly overestimates LPG energy use. For naphtha, the refinery-level
allocation overestimates naphtha energy use. The GHG emission patterns for each of the fuels are similar to those of energy
use.
Conclusions We presented a refining-process-level-based method that can be used to allocate energy use of individual refining processes
to refinery products. The process-level-based method captures process-dependent characteristics of fuel production within
a petroleum refinery. The method starts with the mass and energy flow chart of a refinery, tracks energy use by individual
refining processes, and distributes energy use of a given refining process to products from the process. In allocating energy
use to refinery products, the allocation method could rely on product mass, product energy contents, or product market values
as weighting factors. While the mass- and energy-content-based allocation methods provide an engineering perspective of energy
allocation within a refinery, the market-value-based allocation method provides an economic perspective. The results from
this study show that energy allocations at the aggregate refinery level and at the refining process level could make a difference
in evaluating the energy use and emissions associated with individual petroleum products. Furthermore, for the refining-process-level
allocation method, use of mass — energy content- or market value share-based weighting factors could lead to different results
for diesel fuels, LPG, and naphtha. We suggest that, when possible, energy use allocations should be made at the lowest subprocess
level — a confirmation of the recommendation by the International Standard Organization for life cycle analyses.
Outlook The allocation of energy use in petroleum refineries at the refining process level in this study follows the recommendation
of ISO 14041 that allocations should be accomplished at the subprocess level when possible. We developed a method in this
study that can be readily adapted for refineries in which process-level energy and mass balance data are available. The process-level
allocation helps reveal some additional energy and emission burdens associated with certain refinery products that are otherwise
overlooked with the refinery-level allocation. When possible, process-level allocation should be used in life-cycle analyses. 相似文献
99.
100.
Travis Ingram 《Biological journal of the Linnean Society. Linnean Society of London》2015,116(4):805-818
Body shape variation is integrally related to many aspects of fish ecology, including locomotion and foraging, and can indicate the functional diversity of fish assemblages. Few studies have thoroughly characterized body shape in a diverse marine fish clade, or investigated both temporal and spatial patterns of variation in body shape disparity. Here, I use digital photographs to measure geometric body shape in 66 species of north‐east Pacific rockfish (Sebastes spp.), including a correction for error introduced by arching of specimens. Different components of interspecific shape variation show associations with fish size, depth habitat, trophic niche and phylogenetic relationships. Overall, the accumulation of body shape disparity appears to have been near‐constant over time, and shows little variation across the latitudinal range of rockfish. 相似文献