全文获取类型
收费全文 | 1715篇 |
免费 | 168篇 |
国内免费 | 157篇 |
专业分类
化学 | 427篇 |
晶体学 | 2篇 |
力学 | 102篇 |
综合类 | 25篇 |
数学 | 1133篇 |
物理学 | 351篇 |
出版年
2024年 | 2篇 |
2023年 | 16篇 |
2022年 | 44篇 |
2021年 | 35篇 |
2020年 | 54篇 |
2019年 | 50篇 |
2018年 | 55篇 |
2017年 | 65篇 |
2016年 | 46篇 |
2015年 | 46篇 |
2014年 | 58篇 |
2013年 | 238篇 |
2012年 | 48篇 |
2011年 | 77篇 |
2010年 | 68篇 |
2009年 | 80篇 |
2008年 | 101篇 |
2007年 | 97篇 |
2006年 | 93篇 |
2005年 | 86篇 |
2004年 | 72篇 |
2003年 | 66篇 |
2002年 | 66篇 |
2001年 | 53篇 |
2000年 | 60篇 |
1999年 | 52篇 |
1998年 | 34篇 |
1997年 | 37篇 |
1996年 | 28篇 |
1995年 | 21篇 |
1994年 | 32篇 |
1993年 | 16篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 9篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1974年 | 2篇 |
排序方式: 共有2040条查询结果,搜索用时 31 毫秒
991.
Ziel der Arbeiten war die Untersuchung von Systemvarianten zur simultanen Bestimmung von Gesamtstickstoff und 15N in organischem Probematerial für problemorientierte Anwendungen der 15N-Tracertchnik auf Gebieten der Landwirtschaftsforschung, Biologie, Medizin und Umweltforschung unter Benutzung des emissionsspektroskopischen Analysators NOI-6 zur 15N-Analyse. Von der Vielzahl der prinzipiell verfügbaren kommerziellen Stickstoffanalysatoren konnten folgende Geräte zur Kopplung mit dem NOI-6 eingesetzt werden:
-
—FP 228 der Fa. LECO,
-
—CHN 89.00 der Fa. DANI,
-
—NA 1500 der Fa. Carlo Erba und
-
—Roboprep von der Fa. Europa Scientific.
-
—FP 228 LECO,
-
—CHN 89.00 DANI,
-
—NA 1500 Carlo Erba and
-
—Roboprep Europa Scientific.
992.
Abstract The isotopic compositions of biogenic carbon dioxide and methane from different sites were investigated. The δ13C values of methane vary mainly between ?55‰ and ?75‰ whereas δ13C values of carbon dioxide were found from about + 11‰ to ?23‰. Especially the latter ones are not so typical for microbial gases. The different sites don't vary over the whole scales but form certain groups. Secondary effects like diffusion change the δ values of both components in an even more negative direction, while oxidation processes near the surface result in more positive δ13C values for methane and very negative δ13C values for carbon dioxide. 相似文献
993.
G. Müller 《Isotopes in environmental and health studies》2013,49(2):93-96
For the experimental determination of the H0 2/K ratio of the constants for prototype thermal diffusion columns from initial state mole fraction measurements there was a lack of sufficiently exact simple evaluation methods for polynary isotopic mixtures. The investigation of mole fraction changes at closed ends obtained by numerical integration of the corresponding system of second order partial differential equations resulted in a concept of approximation which allows to utilize approximative analytical solutions originally developed for binary isotopic mixtures. In the approximation the initial mole fractions are indirectly represented by the mean mass number of the initial mixture. Thus, the measured mole fraction changes can be attributed to the difference between the actual mass number of the components and the mean mass number of the enclosed mixture. By simple regression analysis a partial derivative can be obtained which can be attributed to the approximative analytical solutions for binary mixtures. 相似文献
994.
Matthias Barthel Patrick Sturm Alexander Knohl 《Isotopes in environmental and health studies》2013,49(3):359-371
When conducting 13CO2 plant–soil pulse labelling experiments, tracer material might cause unwanted side effects which potentially affect δ13C measurements of soil respiration (δ13CSR) and the subsequent data interpretation. First, when the soil matrix is not isolated from the atmosphere, contamination of the soil matrix with tracer material occurs leading to a physical back-diffusion from soil pores. Second, when using canopy chambers continuously, 13CO2 is permanently re-introduced into the atmosphere due to leaf respiration which then aids re-assimilation of tracer material by the canopy. Accordingly, two climate chamber experiments on European beech saplings (Fagus sylvatica L.) were conducted to evaluate the influence of soil matrix 13CO2 contamination and canopy recycling on soil 13CO2 efflux during 13CO2 plant–soil pulse labelling experiments. For this purpose, a combined soil/canopy chamber system was developed which separates soil and canopy compartments in order to (a) prevent diffusion of 13C tracer into the soil chamber during a 13CO2 canopy pulse labelling and (b) study stable isotope processes in soil and canopy individually and independently. In combination with laser spectrometry measuring CO2 isotopologue mixing ratios at a rate of 1 Hz, we were able to measure δ13C in canopy and soil at very high temporal resolution. For the soil matrix contamination experiment, 13CO2 was applied to bare soil, canopy only or, simultaneously, to soil and canopy of the beech trees. The obtained δ13CSR fluxes from the different treatments were then compared with respect to label re-appearance, first peak time and magnitude. By determining the δ13CSR decay of physical 13CO2 back-diffusion from bare soils (contamination), it was possible to separate biological and physical components in δ13CSR of a combined flux of both. A second pulse labelling experiment, with chambers permanently enclosing the canopy, revealed that 13CO2 recycling at canopy level had no effect on δ13CSR dynamics. 相似文献
995.
The characterisation of 45 Romanian single-strength fruit juices (apples, pears, plums and grapes) collected from different Transylvanian areas by means of stable isotope approach are presented and discussed in this study. We measured 2H/1H, 18O/16O ratios from water juice and 13C/12C from pulp and compared these results with those already reported in the literature for single-strength juices, in order to see how the geographical and climatic conditions of Transylvania and the meteorological peculiarities of the year 2010 influence the isotopic composition of the investigated fruit juices. The δ13C mean values that we found for apple pulp picked up from different Transylvanian areas show slight differences, probably due to the environmental conditions of the plants. No significant correlation either between the variety of apple or the geographical origin and δ13C value was established. 相似文献
996.
Gerhard Strauch Dietmar Haendel Ingeborg Maaß Karl Mühle Annamarie Runge 《Isotopes in environmental and health studies》2013,49(3):280-285
Comparative biochemical and isotope-chemical investigations of cosmopolitical plants open up ways of obtaining parameters from different parts of the Earth which are characterised by variations in the habitat due to different environments. As an Antarctic oasis, the Schirmacher Oasis disposes of adequate favourable ecological conditions for the growth of lower plants. In the present paper, results of isotope studies of lichens, mosses and algae of the Schirmacher Oasis are given and peculiarities of the habitats which influence the isotope contents of the plants are discussed. 相似文献
997.
D. Möcker D. Hofmann K. Jung J. Bender H. J. Weigel 《Isotopes in environmental and health studies》2013,49(2-3):181-190
Abstract Young wheat (C3) and maize (C4) plants were exposed to near-ambient concentrations of ozone in open-top chambers in order to investigate the possible effects of ozone on nitrogen metabolism. Nitrogen was supplied to the plants by adding 15N-labelled tracer substances via the soil substrate. Enzyme activities (NADH nitrate reductase, nitrite reductase, glutamine synthetase and NADH glutamate dehydrogenase) and the incorporation of 15N were determined. The findings show that nitrogen metabolism was affected by O3, however, there were distinct differences between the two species. In plants treated with O3, NADH nitrate reductase activity in maize leaves was reduced, while NR activity in wheat leaves only slightly declined. Only minor changes were observed with respect to the activities of nitrite reductase, glutamine synthetase and NADH glutamate dehydrogenase. Feeding experiments using 15NO3 ? showed that the incorporation of nitrate nitrogen in wheat plants exposed to ozone remains virtually unchanged, whereas in maize plants reduced incorporation rates were observed for nitrate nitrogen. The incorporation of ammonium nitrogen was distinctly increased in wheat and maize by the impact of ozone. When investigating pigment contents, reduced levels of chlorophyll a and b and carotenoids were observed, whereas the pigment content of wheat leaves remained unchanged. These results indicate that young maize plants are more susceptible than wheat plants to short-term ozone exposure. 相似文献
998.
Ergot alkaloids (sum=total alkaloids, TA) originate from the phyto-pathogenic fungus Claviceps purpurea and might exert feed intake depressing and hepatotoxic effects on animals. The aim of the study was to evaluate TA effects on performance and liver function of piglets with the [13C]methacetin breath test and two routes of tracer administration (orally, p.o.; intramuscularly, i.m.). Two ergot batches were mixed into piglet diets resulting in 21 and 17 mg TA kg?1 (Ergot-5 and -12, respectively) and compared with an ergot-free control diet. Feed intake was significantly depressed after feeding the ergot containing diets (p=<0.001). The time at maximum 13CO2 exhalation (t max) and the half-life (t 0.5) were not influenced by treatments and varied between 25 and 68 min after the p.o., and 28 and 62 min after the i.m. administration of [13C]methacetin, respectively. The cumulative 13C recovery (cPDR30) was significantly lower due to feeding the diet Ergot-5 (6.6 %) compared with the Ergot-12 (8.8 %) and the control diet (9.7 %) irrespective of the route of tracer administration (p=0.044). As a discrimination of the diet effects through both tracer administration routes is possible, the i.m. application should be preferred in piglets as this causes less stress than the oral forced administration. 相似文献
999.
H. Faust 《Isotopes in environmental and health studies》2013,49(4):289-326
Abstract A review is being presented of the latest results in the use of nitrogen-15 in the soil-plant system. The literature covers the period from 1991 to early April 1993. The increase in environmentally related experiments shows that the stable nitrogen isotope technique, applied either as tracer or as the natural isotope tracing approach, is well suited particularly to ecosystem studies. This trend is supported by convenient analytical systems. An important challenge to scientists in the field of environmental research is to contrive ways and means of permitting the plants to utilize the available nitrogen in the soil more efficiently. Examples of research areas needing more attention are pointed out. 相似文献
1000.