全文获取类型
收费全文 | 2343篇 |
免费 | 130篇 |
国内免费 | 4篇 |
专业分类
化学 | 2062篇 |
晶体学 | 5篇 |
力学 | 27篇 |
数学 | 212篇 |
物理学 | 171篇 |
出版年
2024年 | 2篇 |
2023年 | 19篇 |
2022年 | 87篇 |
2021年 | 128篇 |
2020年 | 61篇 |
2019年 | 66篇 |
2018年 | 54篇 |
2017年 | 40篇 |
2016年 | 113篇 |
2015年 | 82篇 |
2014年 | 109篇 |
2013年 | 140篇 |
2012年 | 153篇 |
2011年 | 212篇 |
2010年 | 111篇 |
2009年 | 115篇 |
2008年 | 174篇 |
2007年 | 156篇 |
2006年 | 134篇 |
2005年 | 125篇 |
2004年 | 94篇 |
2003年 | 79篇 |
2002年 | 59篇 |
2001年 | 20篇 |
2000年 | 10篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 10篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1972年 | 2篇 |
1938年 | 1篇 |
1933年 | 2篇 |
1932年 | 1篇 |
排序方式: 共有2477条查询结果,搜索用时 656 毫秒
941.
Olliges-Stadler I Stötzel J Koziej D Rossell MD Grunwaldt JD Nachtegaal M Frahm R Niederberger M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(8):2305-2312
Insight into the complex chemical mechanism for the formation of tungstite nanoparticles obtained by the reaction of tungsten hexachloride with benzyl alcohol is presented herein. The organic and inorganic species involved in the formation of the nanoparticles were studied by time-dependent gas chromatography and X-ray diffraction as well as by time-resolved in situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy. Principal component analysis revealed two intermediates, which were identified as WCl(4) and WOCl(4) by using linear combination analysis. Quick-scanning extended X-ray absorption fine structure spectroscopy enabled the time-dependent evolution of the starting compound, the intermediates and the product to be monitored over the full reaction period. The reaction starts with fast chlorine substitution and partial reduction during the dissolution of the tungsten hexachloride in benzyl alcohol followed by the generation of intermediates with W=O double bonds and finally the construction of the W-O-W network of the tungstite structure. 相似文献
942.
Xerogels obtained by drying the gels formed by glucofuranose derivatives with organic solvents were studied. The xerogels were characterized using the SEM, XRD, DSC and OPM techniques. The morphology of a xerogel observed by SEM may change from ‘amorphous, fibrillar’ to ‘crystal-like’, which may be caused by time or temperature. The results suggest that a similar transition may take place when the xerogel is being prepared. Thus SEM pictures of xerogels should be treated with great caution as they may not reflect the gelator network morphology in the bulk gel. 相似文献
943.
This a first step to develop a theory of smooth, étale, and unramified morphisms between Noetherian formal schemes. Our main tool is the complete module of differentials, which is, a coherent sheaf whenever the map of formal schemes is of pseudofinite type. Among our results, we show that these infinitesimal properties of a map of usual schemes carry over into the completion with respect to suitable closed subsets. We characterize unramifiedness by the vanishing of the module of differentials. Also we see that a smooth morphism of Noetherian formal schemes is flat and its module of differentials is locally free. The article closes with a version of Zariski's Jacobian criterion. 相似文献
944.
Marta Kurutz 《Applicable analysis》2013,92(6):603-618
Some loading devices show certain deformation-sensitive but conservative characteristics leading to changes of the classical postbifurcation behavior of structures. To compare the effect of dead and configuration-dependent loading devices, classical bifurcation models are considered. Fully nonlinear behavior is considered, in sense of material, geometry and loading. Originally or approximately nonsmooth functions are included with nonconvexity in the material or loading behaviour. Global equilibrium paths are modified by deformation-sensitive loading devices, nonsmoothness and nonconvexity. 相似文献
945.
Agnieszka Maruszewska Wojciech wiereo Marta Skrka-Majewicz Irena Baranowska-Bosiacka Agata Wszoek Katarzyna Janda Daria Kulis Patrycja Kapczuk Dariusz Chlubek Izabela Gutowska 《Molecules (Basel, Switzerland)》2021,26(14)
Breast milk has the most suitable composition for the proper development in the first year of a child’s life. However, it is often replaced with artificial milk. The aim of the study was to analyze the composition of essential elements: Na, K, Ca, P, Mg, Fe, Zn, Cu, and Mn as well as toxic elements: Ni, Pb, Sr, Li, and In in 18 formulas available in Poland. The daily supply was also estimated. The study was performed by Inductively Coupled Plasma Optical Emission Spectrometry method. The results showed the presence of all essential elements tested, but the content of P and Mn significantly differed from the concentrations declared. Such discrepancies can have significant impact on the daily dose of the bioelements taken. However, the content of elements was within the reference standards established by the EU Directive with exception of P, the amount of which exceeded the norms 5.23–18.80-times. Daily supply of P in tested milk as well as Fe and Mn provided with first and hypoallergenic formula exceeded the adequate intake. Analysis revealed the contamination with harmful elements—Pb, Sr, Li, and In were detected in almost all products. The study confirms the data concerning some discrepancies in composition and the contamination of food and may provide information on the feeding quality of children and estimation of health risk associated with exposure to toxic elements. 相似文献
946.
Marta Borowska 《Entropy (Basel, Switzerland)》2021,23(7)
This paper analyses the complexity of electroencephalogram (EEG) signals in different temporal scales for the analysis and classification of focal and non-focal EEG signals. Futures from an original multiscale permutation Lempel–Ziv complexity measure (MPLZC) were obtained. MPLZC measure combines a multiscale structure, ordinal analysis, and permutation Lempel–Ziv complexity for quantifying the dynamic changes of an electroencephalogram (EEG). We also show the dependency of MPLZC on several straight-forward signal processing concepts, which appear in biomedical EEG activity via a set of synthetic signals. The main material of the study consists of EEG signals, which were obtained from the Bern-Barcelona EEG database. The signals were divided into two groups: focal EEG signals (n = 100) and non-focal EEG signals (n = 100); statistical analysis was performed by means of non-parametric Mann–Whitney test. The mean value of MPLZC results in the non-focal group are significantly higher than those in the focal group for scales above 1 (p < 0.05). The result indicates that the non-focal EEG signals are more complex. MPLZC feature sets are used for the least squares support vector machine (LS-SVM) classifier to classify into the focal and non-focal EEG signals. Our experimental results confirmed the usefulness of the MPLZC method for distinguishing focal and non-focal EEG signals with a classification accuracy of 86%. 相似文献
947.
Plonska-Brzezinska ME Mazurczyk J Palys B Breczko J Lapinski A Dubis AT Echegoyen L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(9):2600-2608
Small multilayer fullerenes, also known as carbon nano-onions (CNOs; 5-6 nm in diameter, 6-8 shells), show higher reactivity than other larger carbon nanostructures. Here we report the first example of an in situ polymerization of aniline on phenyleneamine-terminated CNO surfaces. The green, protonated, conducting emeraldine polyaniline (PANI) was directly synthesized on the surface of the CNO. The functionalized and soluble CNO/PANI composites were characterized by TEM, SEM, DSC, Raman, and infrared spectroscopy. The electrochemical properties of the conducting CNO/PANI films were also investigated. In comparison with pristine CNOs, functionalized carbon nanostructures show dramatically improved solubility in protic solvents, thus enabling their easy processing for coatings, nanocomposites, and biomedical applications. 相似文献
948.
Leporati M Capra P Brizio P Ciccotelli V Abete MC Vincenti M 《Journal of separation science》2012,35(3):400-409
A selective and sensitive method for screening 31 analytes (nine corticosteroids, eight β‐agonists, seven anabolic steroids, six promazines and zeranol) in bovine urine was validated according to 2002/657/EC guidelines. Upon optimization of sample treatment conditions, the extraction was performed by diethylether at pH 9, after deconjugation. Extraction yields (R%) proved higher than 70% for 19 analytes, 50<R%<70 for 5 analytes, lower than 50% but reproducible for the remaining six analytes. The analyses were carried out using HPLC‐ESI‐MS/MS. The method sensitivity proved high enough to largely exceed the CCβ requirements of the Italian residue detection plan, ranging from 1 to 3 ng/mL (20 ng/mL for promazines). The present method allowed the simultaneous analysis of most drugs for which the European legislation prescribes official controls. Its practical applicability was verified on 494 real samples as an alternative to the traditional screening protocols based on multiple immunometric analysis, demonstrating high efficiency and comprehensive investigation capacity, allowing epidemiological assessment of the current trends in cattle breeding drug abuse. Among non‐compliant results, nine borderline cases of growth‐promoters illegal treatments, making use of long‐term low‐dosage administrations and typically yielding urine residues below the cut‐off value for immunochemical methods, were detected by using the present LC‐MS/MS method. 相似文献
949.
The development of DNA-based methods for the identification and quantification of fish in food and feed samples is frequently focused on a specific fish species and/or on the detection of mitochondrial DNA of fish origin. However, a quantitative method for the most common fish species used by the food and feed industry is needed for official control purposes, and such a method should rely on the use of a single-copy nuclear DNA target owing to its more stable copy number in different tissues. In this article, we report on the development of a real-time PCR method based on the use of a nuclear gene as a target for the simultaneous detection of fish DNA from different species and on the evaluation of its quantification potential. The method was tested in 22 different fish species, including those most commonly used by the food and feed industry, and in negative control samples, which included 15 animal species and nine feed ingredients. The results show that the method reported here complies with the requirements concerning specificity and with the criteria required for real-time PCR methods with high sensitivity. 相似文献
950.
Sanchís J Kantiani L Llorca M Rubio F Ginebreda A Fraile J Garrido T Farré M 《Analytical and bioanalytical chemistry》2012,402(7):2335-2345
Despite having been the focus of much attention from the scientific community during recent years, glyphosate is still a challenging
compound from an analytical point of view because of its physicochemical properties: relatively low molecular weight, high
polarity, high water solubility, low organic solvent solubility, amphoteric behaviour and ease to form metal complexes. Large
efforts have been directed towards developing suitable, sensitive and robust methods for the routine analysis of this widely
used herbicide. In the present work, a magnetic particle immunoassay (IA) has been evaluated for fast, reliable and accurate
part-per-trillion monitoring of glyphosate in water matrixes, in combination with a new analytical method based on solid-phase
extraction (SPE), followed by liquid chromatography (LC) coupled to tandem mass spectrometry (MS/MS), for the confirmatory
analysis of positive samples. The magnetic particle IA has been applied to the analysis of about 140 samples of groundwater
from Catalonia (NE Spain) collected during four sampling campaigns. Glyphosate was present above limit of quantification levels
in 41% of the samples with concentrations as high as 2.5 μg/L and a mean concentration of 200 ng/L. Good agreement was obtained
when comparing the results from IA and on-line SPE-LC-MS/MS analyses. In addition, no false negatives were obtained by the
use of the rapid IA. This is one of the few works related to the analysis of glyphosate in real groundwater samples and the
presented data confirm that, although it has low mobility in soils, glyphosate is capable of reaching groundwater. 相似文献