全文获取类型
收费全文 | 31247篇 |
免费 | 2636篇 |
国内免费 | 3637篇 |
出版年
2024年 | 66篇 |
2023年 | 658篇 |
2022年 | 682篇 |
2021年 | 987篇 |
2020年 | 1071篇 |
2019年 | 1414篇 |
2018年 | 1105篇 |
2017年 | 1169篇 |
2016年 | 1222篇 |
2015年 | 1297篇 |
2014年 | 1668篇 |
2013年 | 2227篇 |
2012年 | 1280篇 |
2011年 | 1516篇 |
2010年 | 1243篇 |
2009年 | 1629篇 |
2008年 | 1701篇 |
2007年 | 1688篇 |
2006年 | 1564篇 |
2005年 | 1449篇 |
2004年 | 1289篇 |
2003年 | 1142篇 |
2002年 | 1036篇 |
2001年 | 815篇 |
2000年 | 704篇 |
1999年 | 755篇 |
1998年 | 590篇 |
1997年 | 556篇 |
1996年 | 507篇 |
1995年 | 503篇 |
1994年 | 490篇 |
1993年 | 375篇 |
1992年 | 349篇 |
1991年 | 338篇 |
1990年 | 268篇 |
1989年 | 249篇 |
1988年 | 229篇 |
1987年 | 210篇 |
1986年 | 168篇 |
1985年 | 191篇 |
1984年 | 203篇 |
1983年 | 141篇 |
1982年 | 168篇 |
1981年 | 115篇 |
1980年 | 116篇 |
1979年 | 105篇 |
1978年 | 81篇 |
1977年 | 46篇 |
1976年 | 42篇 |
1974年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
In producing power, humans move the nutrients nitrogen (N) and phosphorus (P) from their long‐term geological and biological stocks and release or emit them in soil, water, and the atmosphere. In Finland, peat combustion is an important driver of N and P fluxes from the environment to human economy. The flows of N and P in the Finnish energy system were quantified with partial substance flow analysis, and the driving forces of emissions of nitrogen oxides (NOx) were analyzed using the ImPACT model. In the year 2000 in Finland, 140,000 tonnes of nitrogen entered the energy system, mainly in peat and hard coal. Combustion released an estimated 66,000 tonnes of N as nitrogen oxides (NOx) and nitrous oxides (N2O) and another 74,000 tonnes as elemental N2. Most of the emissions were borne in traffic. At the same time, 6,000 tonnes of P was estimated to enter the Finnish energy system, mostly in peat and wood. Ash was mainly used in earth construction and disposed in landfills; thus negligible levels of P were recycled back to nature. During the twentieth century, fuel‐borne input of N increased 20‐fold, and of P 8‐fold. In 1900–1950, the increasing use of hard coal slowly boosted N input, whereas wood fuels were the main carrier of P. Since 1970, the fluxes have been on the rise. NOx emissions leveled off in the 1980s, though, and then declined in conjunction with improvements in combustion technologies such as NOx removal (de‐NOx) technologies in energy production and catalytic converters in cars. 相似文献
92.
93.
94.
A statistical test is described to verify the characteristics of the biological information contained in the dynamics of the flowering process. The test focuses on interactions between the pollen index and climatic variables to investigate if the biological indicator can synthesise the information of the pre-flowering phases. The multiple-regression model is built upon two pre-flowering climate macro-indicators extracted by Principal Component Analysis (PCA) and the optimised pollen index is obtained by non-parametric estimation. The empirical analysis is applied to 15 stations located in southern Italy in regions that have a longstanding tradition of olive production. Using the variance explained, we find that an optimised pollen index is fairly well predicted by the pre-flowering climatic data. We conclude that the optimised pollen index makes more parsimonious the modelling for predicting olive production. 相似文献
95.
The gill surface area of 0-group, post-metamorphic Pleuronectes flesus L. was examined using digital image analysis software and expressed in relation to body mass according to the equation log Y=loga+c logW ( a =239·02; c =0·723). The components that constitute gill area, total filament length, interlamellar space and unilateral lamellar area were measured. The measurement of the length of every filament on all eight arches showed that commonly used methods of calculation can lead to an under-estimation of up to 24% of total filament length. Direct measurements of unilateral lamellar area with digital image analysis showed that previously reported gill area data for the same species was over-estimated by as much as 58%. In addition, in this species the neglect of gill pouch asymmetry after metamorphosis, can bring about a 14% over-estimation of total gill area. 相似文献
96.
H. Daniel 《Biological trace element research》1987,13(1):301-318
Because of the large μ mass compared to the electron mass, the muonic X-rays have energies very suitable for standard γ-ray
spectroscopy (Ge detectors), so every element is easily recognized. By selecting the primary μ energies appropriately any
part of the specimen, also well inside, can be nondestructively investigated. On the other hand, surface layers may be analyzed.
Accuracies of quantitative analyses are 1% of the atomic abundance of the element in question in favorite cases. Results on
applications in nuclear medicine and surface physics are presented, and ways of improving the muon flux density are discussed. 相似文献
97.
Many biomedical experiments require the qualitative and quantitative localization of trace elements with high sensitivity
and good spatial resolution. The feasibility of measuring the chemical form of the elements, the time course of trace element
metabolism, and conducting experiments in living biological systems are also important requirements for biological trace element
research. Nuclear analytical techniques that employ ion or photon beams have grown in importance in the past decade and have
led to several new experimental approaches. Some of the important features of these methods are reviewed here along with their
role in trace element research. Examples of their use are given to illustrate potential for new research directions. It is
emphasized that the effective application of these methods necessitates a closely integrated multidisciplinary scientific
team. 相似文献
98.
Paternal inheritance of chloroplast DNA in Larix 总被引:2,自引:0,他引:2
Restriction enzyme analysis was used to determine the inheritance of chloroplast DNA in conifers. The plant material studied included five individual trees of European larch (Larix decidua Mill.) and Japanese larch (Larix leptolepis Sieb. & Zucc.) and six hybrids from controlled crosses between these species. The chloroplast DNA fragment patterns generated by Bam-HI and Bcl-I were species-specific. Paternal inheritance of chloroplast DNA patterns was found in most Larix crosses. One hybrid showed maternal chloroplast DNA patterns. In addition, two other hybrids had mixed Bam-HI patterns suggesting recombination between maternal and paternal chloroplast DNA. The mechanisms favoring paternal inheritance in conifers are not known. Paternal inheritance of chloroplast DNA is suggested it to be a general phenomenon in conifers. 相似文献
99.
100.
D. GODFREY J. N. LYTHGOE† ‡ D. A. RUMBALL § 《Biological journal of the Linnean Society. Linnean Society of London》1987,32(4):427-433
Some striped animals are camouflaged in their natural environment, whereas others are conspicuous. Mammals are known to have spatial frequency analysers in their visual mechanism, and it is suggested that the spatial characteristics of a striped pattern are different in camouflaged and conspicuous animals. Fourier analysis of the stripes of the zebra shows spatial frequencies in the pattern that are unlikely to be present so strongly in their natural background scene. A similar analysis of the camouflaging stripes of a tiger show that the distribution of spatial frequencies are similar to that in the background scene. 相似文献