全文获取类型
收费全文 | 2117篇 |
免费 | 82篇 |
国内免费 | 35篇 |
出版年
2024年 | 6篇 |
2023年 | 34篇 |
2022年 | 26篇 |
2021年 | 46篇 |
2020年 | 51篇 |
2019年 | 87篇 |
2018年 | 70篇 |
2017年 | 75篇 |
2016年 | 76篇 |
2015年 | 45篇 |
2014年 | 91篇 |
2013年 | 156篇 |
2012年 | 50篇 |
2011年 | 129篇 |
2010年 | 70篇 |
2009年 | 125篇 |
2008年 | 112篇 |
2007年 | 136篇 |
2006年 | 105篇 |
2005年 | 88篇 |
2004年 | 75篇 |
2003年 | 48篇 |
2002年 | 44篇 |
2001年 | 43篇 |
2000年 | 30篇 |
1999年 | 30篇 |
1998年 | 40篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 32篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 18篇 |
1990年 | 25篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1974年 | 3篇 |
1971年 | 1篇 |
排序方式: 共有2234条查询结果,搜索用时 250 毫秒
41.
Claude E. Boyd 《Hydrobiologia》1981,80(1):91-93
Ion-pairing has little effect on ionic activity calculations for weakly mineralized natural water (ionic strength < 2 mM). However, for more strongly mineralized freshwaters, corrections for ion-pairing are necessary if highly accurate ionic activities are required. 相似文献
42.
43.
44.
Leonard Robert Gardner 《Biogeochemistry》1990,11(2):97-110
Residual soils (saprolites) developed on crystalline rocks appear to form by an essentially isovolumetric process (i.e. without dilation or compaction). Isovolumetric geochemical analysis of a suite of saprolite samples developed on a common parent rock can be used to estimate the relative rates of long-term losses of P and Si during weathering. Using the export of dissolved Si in rivers as a weathering index, one can then estimate the rate of P release due to chemical weathering by means of the P-Si loss ratio in saprolite. For three basins where data are available (Liberty Hill, SC; Amazon River, Brazil: Rio Negro, Brazil) estimated P weathering release rates are 163, 457, and 242 moles P km–2 yr–1 respectively. These compare to precipitation inputs of 684, 700 and 630 moles P km–2 yr–1 and total river exports of 256, 4490 and 820 moles P km–2 yr–1, respectively. The Rio Negro shows a near perfect balance between the input of P via precipitation and chemical weathering and the riverine output of dissolved and suspended P. This system, however, raised the unsolved problem of the source that supports the atmospheric P input. 相似文献
45.
Tomasz Frączyk 《化学与生物多样性》2021,18(4):e2100043
Proteins anchor copper(II) ions mainly by imidazole from histidine residues located in different positions in the primary protein structures. However, the motifs with histidine in the first three N-terminal positions (His1, His2, and His3) show unique Cu(II)-binding properties, such as availability from the surface of the protein, high flexibility, and high Cu(II) exchangeability with other ligands. It makes such sequences beneficial for the fast exchange of Cu(II) between ligands. Furthermore, sequences with His1 and His2, thus, non-saturating the Cu(II) coordination sphere, are redox-active and may play a role in Cu(II) reduction to Cu(I). All human protein sequences deposited in UniProt Knowledgebase were browsed for those containing His1, His2, or His3. Proteolytically modified sequences (with the removal of a propeptide or Met residue) were taken for the analysis. Finally, the sequences were sorted out according to the subcellular localization of the proteins to match the respective sequences with the probability of interaction with divalent copper. 相似文献
46.
N Ivalú Cacho Patrick J McIntyre Daniel J Kliebenstein Sharon Y Strauss 《Annals of botany》2021,127(7):887
Background and AimsWe investigate patterns of evolution of genome size across a morphologically and ecologically diverse clade of Brassicaceae, in relation to ecological and life history traits. While numerous hypotheses have been put forward regarding autecological and environmental factors that could favour small vs. large genomes, a challenge in understanding genome size evolution in plants is that many hypothesized selective agents are intercorrelated.MethodsWe contribute genome size estimates for 47 species of Streptanthus Nutt. and close relatives, and take advantage of many data collections for this group to assemble data on climate, life history, soil affinity and composition, geographic range and plant secondary chemistry to identify simultaneous correlates of variation in genome size in an evolutionary framework. We assess models of evolution across clades and use phylogenetically informed analyses as well as model selection and information criteria approaches to identify variables that can best explain genome size variation in this clade.Key ResultsWe find differences in genome size and heterogeneity in its rate of evolution across subclades of Streptanthus and close relatives. We show that clade-wide genome size is positively associated with climate seasonality and glucosinolate compounds. Model selection and information criteria approaches identify a best model that includes temperature seasonality and fraction of aliphatic glucosinolates, suggesting a possible role for genome size in climatic adaptation or a role for biotic interactions in shaping the evolution of genome size. We find no evidence supporting hypotheses of life history, range size or soil nutrients as forces shaping genome size in this system.ConclusionsOur findings suggest climate seasonality and biotic interactions as potential forces shaping the evolution of genome size and highlight the importance of evaluating multiple factors in the context of phylogeny to understand the effect of possible selective agents on genome size. 相似文献
47.
Xiaohui Zeng Jiatu Liu Jianfeng Mao Junnan Hao Zhijie Wang Si Zhou Chris D. Ling Zaiping Guo 《Liver Transplantation》2020,10(32)
Rechargeable aqueous Zn/MnO2 batteries are very attractive large‐scale energy storage technologies, but still suffer from limited cycle life and low capacity. Here the novel adoption of a near‐neutral acetate‐based electrolyte (pH ≈ 6) is presented to promote the two‐electron Mn4+/Mn2+ redox reaction and simultaneously enable a stable Zn anode. The acetate anion triggers a highly reversible MnO2/Mn2+ reaction, which ensures high capacity and avoids the issue of structural collapse of MnO2. Meanwhile, the anode‐friendly electrolyte enables a dendrite‐free Zn anode with outstanding stability and high plating/stripping Coulombic efficiency (99.8%). Hence, a high capacity of 556 mA h g?1, a lifetime of 4000 cycles without decay, and excellent rate capability up to 70 mA cm?2 are demonstated in this new near‐neutral aqueous Zn/MnO2 battery by simply manipulating the salt anion in the electrolyte. The acetate anion not only modifies the surface properties of MnO2 cathode but also creates a highly compatible environment for the Zn anode. This work provides a new opportunity for developing high‐performance Zn/MnO2 and other aqueous batteries based on the salt anion chemistry. 相似文献
48.
Jrg Prietzel Wolfgang Falk Birgit Reger Enno Uhl Hans Pretzsch Lothar Zimmermann 《Global Change Biology》2020,26(10):5796-5815
At two forest sites in Germany (Pfaffenwinkel, Pustert) stocked with mature Scots pine (Pinus sylvestris L.), we investigated changes of topsoil chemistry during the recent 40 years by soil inventories conducted on replicated control plots of fertilization experiments, allowing a statistical analysis. Additionally, we monitored the nutritional status of both stands from 1964 until 2019 and quantified stand growth during the monitoring period by repeated stand inventories. Moreover, we monitored climate variables (air temperature and precipitation) and calculated annual climatic water balances from 1991 to 2019. Atmospheric nitrogen (N) and sulfur (S) deposition between 1964 and 2019 was estimated for the period 1969–2019 by combining annual deposition measurements conducted in 1985–1987 and 2004 with long‐term deposition records from long‐term forest monitoring stations. We investigated interrelations between topsoil chemistry, stand nutrition, stand growth, deposition, and climate trends. At both sites, the onset of the new millennium was a turning point of important biogeochemical processes. Topsoil acidification turned into re‐alkalinization, soil organic matter (SOM) accumulation stopped, and likely turned into SOM depletion. In the new millennium, topsoil stocks of S and plant‐available phosphorus (P) as well as S and P concentrations in Scots pine foliage decreased substantially; yet, age‐referenced stand growth remained at levels far above those expected from yield table data. Tree P and S nutrition as well as climate change (increased temperature and drought stress) have replaced soil acidification as major future challenges for both forests. Understanding of P and S cycling and water fluxes in forest ecosystems, and consideration of these issues in forest management is important for successfully tackling the new challenges. Our study illustrates the importance of long‐term forest monitoring to identify slow, but substantial changes of forest biogeochemistry driven by natural and anthropogenic global change. 相似文献
49.
Dye removal from industrial waste water has become an important issue. The highvisibility, undesirability and recalcitrance are the significant environmental problemfor the dyes. In the present work,β‐cyclodextrin functionalized KCC‐1 (KCC‐1‐NH‐β‐CD)was synthesized and utilized to the removal of hazardous malachite green. In order to study the morphology of the synthesized nano adsorbent, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were obtained from the surface of the sample. Additionally, the functionalization of KCC‐1 with β‐cyclodextrin was confirmed with Furrier Transform Infrared spectroscopy (FTIR). The textural property of KCC‐1 was verified using nitrogen adsorption/ desorption analysis (BET equation). UV‐Vis spectroscopy utilized for the investigation of malachite green by KCC‐1‐NH‐β‐CD. Specific surface area of the adsorbent was calculated to be 140 m2/g and it can be stated that the synthesized nano adsorbent has high removal efficiency. It should be noted that the adsorption capacity of the employed nano adsorbent was more than 95%, which could be attributed to high porosity of β‐cyclodextrin functionalized KCC‐1. 相似文献
50.
《Bioorganic & medicinal chemistry letters》2020,30(19):127434
In the present study, a series of new isoniazid embedded triazole derivatives have been synthesized. These compounds were evaluated for their in vitro antitubercular and antimicrobial activities. Among the screened compounds, six have exhibited potent antitubercular activity against Mycobacterium tuberculosis H37Rv strain with MIC value 0.78 μg/mL, whereas, three compounds have displayed activity with MIC value ranging from 1.56 to 3.125 μg/mL. The cytotoxicity of the active compounds was studied against RAW 264.7 cell line by MTT assay and no toxicity was observed even at 25 μg/mL concentration. The five compounds have displayed good antimicrobial activities. Molecular docking have been performed against mycobacterial InhA enzyme to gain an insight into the plausible mechanism of action which could pave the way for our endeavor to identify potent antitubercular candidates. We believe that further optimization of these molecules may lead to potent antitubercular agents. 相似文献