全文获取类型
收费全文 | 2572篇 |
免费 | 65篇 |
国内免费 | 9篇 |
专业分类
化学 | 1693篇 |
晶体学 | 26篇 |
力学 | 58篇 |
数学 | 307篇 |
物理学 | 562篇 |
出版年
2023年 | 17篇 |
2022年 | 79篇 |
2021年 | 81篇 |
2020年 | 55篇 |
2019年 | 56篇 |
2018年 | 53篇 |
2017年 | 41篇 |
2016年 | 77篇 |
2015年 | 87篇 |
2014年 | 82篇 |
2013年 | 153篇 |
2012年 | 162篇 |
2011年 | 175篇 |
2010年 | 111篇 |
2009年 | 108篇 |
2008年 | 154篇 |
2007年 | 122篇 |
2006年 | 108篇 |
2005年 | 105篇 |
2004年 | 88篇 |
2003年 | 55篇 |
2002年 | 72篇 |
2001年 | 44篇 |
2000年 | 43篇 |
1999年 | 40篇 |
1998年 | 35篇 |
1997年 | 34篇 |
1996年 | 29篇 |
1995年 | 31篇 |
1994年 | 32篇 |
1993年 | 27篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1988年 | 23篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 27篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 9篇 |
1973年 | 6篇 |
排序方式: 共有2646条查询结果,搜索用时 15 毫秒
221.
Silva R Boldt S Costa VM Carmo H Carvalho M Carvalho F Bastos Mde L Lemos-Amado F Remião F 《Biomedical chromatography : BMC》2007,21(7):670-679
The sustained high release of catecholamines to circulation is a deleterious condition that may induce toxicity, which seems to be partially related to the products formed by oxidation of catecholamines that can be further conjugated with glutathione (GSH). The aim of the present study was to develop a method for the determination of GSH adducts of adrenaline in biological samples. Two position isomers of the glutathion-S-yl-adrenaline were synthesized and characterized by HPLC using diode array, coulometric and mass detectors. A method for the extraction of these adducts from human plasma was also developed, based on adsorption to activated alumina, which showed adequate recoveries and proved to be crucial in removing interferences from plasma. The selectivity, precision and linearity of the method were all within the accepted values for these parameters. Furthermore, the sensitivity of this method allows the detection of adduct amounts that are within the range of the expected concentrations for these adducts under certain pathophysiological conditions and/or drug treatments. In conclusion, the development of this method allows the direct analysis of GSH adducts of adrenaline in human plasma, providing a valuable tool for the study of the catecholamine oxidation process and its related toxicity. 相似文献
222.
Alysson M. Costa Jean-François Cordeau Bernard Gendron 《Computational Optimization and Applications》2009,42(3):371-392
Solving multicommodity capacitated network design problems is a hard task that requires the use of several strategies like
relaxing some constraints and strengthening the model with valid inequalities. In this paper, we compare three sets of inequalities
that have been widely used in this context: Benders, metric and cutset inequalities. We show that Benders inequalities associated
to extreme rays are metric inequalities. We also show how to strengthen Benders inequalities associated to non-extreme rays
to obtain metric inequalities. We show that cutset inequalities are Benders inequalities, but not necessarily metric inequalities.
We give a necessary and sufficient condition for a cutset inequality to be a metric inequality. Computational experiments
show the effectiveness of strengthening Benders and cutset inequalities to obtain metric inequalities. 相似文献
223.
Paulo Augusto da Costa Filho 《Analytica chimica acta》2009,631(2):206-461
This paper reports the results of a rapid method to determine sucrose in chocolate mass using near infrared spectroscopy (NIRS). We applied a broad-based calibration approach, which consists in putting together in one single calibration samples of various types of chocolate mass. This approach increases the concentration range for one or more compositional parameters, improves the model performance and requires just one calibration model for several recipes. The data were modelled using partial least squares (PLS) and multiple linear regression (MLR). The MLR models were developed using a variable selection based on the coefficient regression of PLS and genetic algorithm (GA). High correlation coefficients (0.998, 0.997, 0.998 for PLS, MLR and GA-MLR, respectively) and low prediction errors confirms the good predictability of the models. The results show that NIR can be used as rapid method to determine sucrose in chocolate mass in chocolate factories. 相似文献
224.
225.
Flávio A. Pavan Yoshitaka Gushikem Celso C. Moro Tania M. Costa Edilson V. Benvenutti 《Colloid and polymer science》2003,281(2):173-177
Anilinepropylsilica xerogel was obtained by using an appropriate organosilane and tetraethyl orthosilicate as precursor reagents. The gelation was carried out using HF and NaF as catalysts. The presence of Na+ (when NaF was used) resulted in a decrease in the final organic content of the materials. This effect was interpreted as an inhibition of the organosilane polycondensation possibly due to the Na+ interaction with the SiO- groups of the hydrolyzed organosilane. The presence of Na+ also results in morphological changes in the xerogels. 相似文献
226.
227.
R. Carlini D. Macciò M. Pani N. Parodi G. Zanicchi M. M. Carnasciali G. A. Costa 《Journal of Thermal Analysis and Calorimetry》2013,112(1):513-517
The substitution of Sb with As in the NiSbS intermetallic compound was studied in the framework of evaluating a possible increase of the thermoelectric properties. Different NiSb1?xAsxS samples were synthesized with increasing amounts of As (0 < x < 0.66) employing a simple synthetic route using a muffle furnace. Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy was used to investigate the microstructure. X-ray powder diffraction techniques were employed in order to study the possible existence of a solid solution between NiSbS and NiAsS compounds, as well as to identify the crystal structure and determine the lattice parameters. All compounds were found to crystallise with the NiSbS prototype (cP12-P213), with lattice parameters varying from a = 0.59341(7) nm (x = 0) to a = 0.56849(6) nm (x = 1). Good agreement with Vegard’s law was evidenced. Thermal measurements on NiSb1?xAsxS samples were carried out using DTA instruments to evaluate the thermal stability and the melting temperatures. 相似文献
228.
A first step in the thermal degradation of polystyrene prepared by radical polymerisation has been isolated by heating the polymer in the temperature range 199–280°C. In this step a chain scission process occurs without formation of volatile products, involving, on average, about one bond between structural units in every 10 000. This gives more direct evidence than hitherto of the presence of ‘weak links’ in polystyrene which are shown to be randomly distributed in the polymer chains, their scission resulting in a single break in the molecule of polystyrene to which they belong.The very low energy of activation for chain scission suggests that more than one rate determining step is involved in the process. 相似文献
229.
The Cr 2p and O 1s binding energy (BE) levels have been calculated by first principles methods for different models of hydroxylated (0001)-Cr2O3 surfaces. Several surface terminations have been considered. The calculations allow us to reproduce the O 1s shifts between O in oxide and OH groups. It is found that two main effects account for the OH binding energy shifts. On the one hand, the increased covalency of the O–H bond with respect to the Cr–O bond, lowers the electronic O (1s and 2p) energy, and in consequence the BE of the core levels (O 1s) are higher. On the other hand, the lower the OH coordination number, the higher the valence and core levels energy, and the lower the BE. Consequently, mono-coordinated hydroxyls have a binding energy near that of O2? in the oxide (ΔBEOH–O = ? 0.2–0.0 eV). Two-fold coordinated hydroxyls have a slightly higher BE (ΔBEOH–O = + 0.3 eV). Three-fold coordinated OH groups have a higher binding energy (ΔBEOH–O = + 0.6?0.7 eV), corresponding to that experimentally measured for OH groups. Finally, water adsorbed above OH groups exhibits a still higher BE (ΔBEHOH–O = + 0.9–1.0 eV). The ΔBE are slightly under-estimated under the initial state approximation, and overestimated under the final state (Z + 1) approximation. 相似文献
230.
V. Costa K. Leyssens A. Adriaens N. Richard F. Scholz 《Journal of Solid State Electrochemistry》2010,14(3):449-451
The characterization of materials constituting cultural artefacts is a challenging step in their conservation, due to the
object’s uniqueness and the reduced number of conservation institutes able to supply non-destructive analysis. We propose
an alternative analytical tool, which combines accessibility (low cost and portable) and high sensitivity, based on electrochemical
linear sweep voltammetry (LSV) with paraffin impregnated graphite electrode (PIGE). To investigate the composition of “white
alloys” that certainly have been used as decoration on copper-based Roman fibulae, sampling was done very locally by gently
rubbing the selected areas with the PIGE. LSV results evidence the presence of silver, lead, and tin, supporting the argument
provided by typological analysis that these metals were used for decoration. 相似文献