全文获取类型
收费全文 | 1335篇 |
免费 | 26篇 |
国内免费 | 8篇 |
专业分类
化学 | 769篇 |
晶体学 | 34篇 |
力学 | 45篇 |
数学 | 353篇 |
物理学 | 168篇 |
出版年
2023年 | 5篇 |
2022年 | 7篇 |
2021年 | 20篇 |
2020年 | 16篇 |
2019年 | 20篇 |
2018年 | 11篇 |
2017年 | 24篇 |
2016年 | 33篇 |
2015年 | 22篇 |
2014年 | 30篇 |
2013年 | 68篇 |
2012年 | 62篇 |
2011年 | 83篇 |
2010年 | 50篇 |
2009年 | 58篇 |
2008年 | 68篇 |
2007年 | 72篇 |
2006年 | 74篇 |
2005年 | 64篇 |
2004年 | 66篇 |
2003年 | 54篇 |
2002年 | 54篇 |
2001年 | 22篇 |
2000年 | 29篇 |
1999年 | 21篇 |
1998年 | 20篇 |
1997年 | 11篇 |
1996年 | 24篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 18篇 |
1985年 | 26篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 19篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1913年 | 2篇 |
1907年 | 2篇 |
排序方式: 共有1369条查询结果,搜索用时 31 毫秒
51.
Dr. Mateusz Janeta Prof. Tadeusz Lis Prof. Sławomir Szafert 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(60):13686-13697
In the present research, the synthesis, spectroscopic characterization, and structural investigations of a unique ZnII complex of imine-functionalized polyhedral oligomeric silsesquioxane (POSS) is designed, and hereby described, as a catalyst for the synthesis of cyclic carbonates from epoxides and CO2. The uncommon features of the designed catalytic system is the elimination of the need for a high pressure of CO2 and the significant shortening of reaction times commonly associated with such difficult transformations like that of styrene oxide to styrene carbonate. Our studies have shown that imine-POSS is able to chelate metal ions like ZnII to form a unique coordination complex. The silsesquioxane core and the hindrance of the side arms (their steric effect) influence the construction process of the homoleptic Zn4@POSS-1 complex. The compound was characterized in solution by NMR (1H, 13C, 29Si), ESI-MS, UV/Vis spectroscopy and in the solid state by thermogravimetric/differential thermal analysis (TG-DTA), elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), cross-polarization magic angle spinning (CP MAS) NMR (13C, 29Si) spectroscopy, and X-ray crystallography. 相似文献
52.
53.
Fast separations are very desirable in laboratories that analyze large numbers of samples per day or those needing short turn‐around times. Traditional HPLC methods using conventional stationary phases and standard column dimensions require significant amounts of organic solvents and generate large volumes of waste. With growing awareness about the environment, the development of green technologies has been receiving increasing attention. In this work, a very fast green analytical method based on LC‐UV using a short narrow bore column packed with fully porous sub‐2 μm particles has been developed for simultaneous determination of nine pharmaceuticals in wastewater and surface water. The chromatographic separation was optimized in order to achieve short analysis time and good resolution for all analytes in a single run. All analytes could be separated in 1 min with good resolution. Sample preparation was executed by solid phase extraction using Oasis HLB cartridges. The method developed was validated based on parameters such as linearity, precision, accuracy, detection, and quantification limits. The recovery ranged from 70.9 to 92.5% with SDs not higher than 5.4%, except for acetaminophen and sulphanilamide. LODs ranged from 0.6–2.5 μg/L, while the LOQs were in the range 2–8 μg/L. 相似文献
54.
The determination of quercetin and rutin by flow injection analysis (FIA) and capillary electrophoresis (CE) using electrochemical detection was described. These flavonoids were determined at normal (unheated) and hot platinum microelectrodes using cyclic voltammetry. When quercetin or rutin is reaching the platinum electrode, a change of the current in the region of the platinum oxide formation is observed. Integration of the current changes in this in this region creates analytical signals in the form of peaks. An increase of temperature to about 76 ?C in a small zone adjacent to the microelectrode causes an increase of the analytical signal by more than 6 times under FIA conditions. This method enables the use of hot microelectrodes as detectors in HPLC or CE. In CE the improvement of the analytical signal at hot microelectrodes is smaller than in FIA and increase only 1.3–3.4 times. Heated microelectrodes were used for analysis of the flavonoids in natural samples of the plant (extract of sea buckthorn) and a pharmaceutical preparation (Cerutin). 相似文献
55.
The retention of aliphatic hydrocarbons with polar groups has been compared in respect to the separation selectivity changes in reversed-phase high-performance liquid chromatography with C18 stationary phase type and binary water eluent composed of methanol, acetonitrile, or tetrahydrofuran as modifiers. The changes in separation selectivity when one modifier is replaced by another in the eluent is explained, taking into consideration molecular interactions of the solutes with components of the stationary phase region, i.e., extracted modifier, and ordering of the stationary phase by the modifier.
相似文献56.
Herbicidal ionic liquid with dual functions has been synthesized and characterized (properties, chemical stability and biological activity). 2-Chloroethyltrimethylammonium (4-chloro-2-methylphenoxy)acetate prepared in an alkaline environment undergoes the reaction of dehydrochlorination by the E2 mechanism. This may be described as a specific instance of providing third generation ILs with high potential of application, which preserve both pesticidal properties: the herbicidal activity due to their anions and plant growth inhibition effect due to their cations. 相似文献
57.
Tracy Zhang Bob A. Howell Patrick B. Smith 《Journal of Thermal Analysis and Calorimetry》2014,116(3):1369-1378
Hyperbranched poly(ester)s offer attractive features for applications in a number of areas, particularly as platforms for the support of controlled release actives in the agricultural and biomedical fields. Such materials have been generated from trimethylolpropane and adipic acid, and fully characterized using chromatographic, spectroscopic, and thermal methods. The thermal stability of these polymers has been assessed using thermogravimetry and infrared spectroscopy. The degradation characteristics of these materials have been compared to those of two linear adipic acid polymers. The prominent feature of the thermal degradation of the hyperbranched poly(ester)s is ether formation while that for the comparable linear poly(ester)s is ester pyrolysis resulting in chain scission. 相似文献
58.
Barbara Burnat Tadeusz Blaszczyk Andrzej Leniart 《Journal of Solid State Electrochemistry》2014,18(11):3111-3119
Stainless steel ISO 5832–9 type is often used to perform implants which operate in protein-containing physiological environments. The interaction between proteins and surface of the implant may affect its corrosive properties. The aim of this work was to study the effect of selected serum proteins (albumin and γ-globulins) on the corrosion of ISO 5832–9 alloy (trade name M30NW) which surface was modified by titania coatings. These coatings were obtained by sol–gel method and heated at temperatures of 400 and 800 °C. To evaluate the effect of the proteins, the corrosion tests were performed with and without the addition of proteins with concentration of 1 g L?1 to the physiological saline solution (0.9 % NaCl, pH 7.4) at 37 °C. The tests were carried out within 7 days. The following electrochemical methods were used: open circuit potential, linear polarization resistance, and electrochemical impedance spectroscopy. In addition, surface analysis by optical microscopy and X-ray photoelectron spectroscopy (XPS) method was done at the end of weekly corrosion tests. The results of corrosion tests showed that M30NW alloy both uncoated and modified with titania coatings exhibits a very good corrosion resistance during weekly exposition to corrosion medium. The best corrosion resistance in 0.9 % NaCl solution is shown by alloy samples modified by titania coating annealed at 400 °C. The serum proteins have no significant effect onto corrosion of investigated biomedical steel. The XPS results confirmed the presence of proteins on the alloy surface after 7 days of immersion in protein-containing solutions. 相似文献
59.
Gyroscope‐Like Molecules Consisting of PdX2/PtX2 Rotators within Three‐Spoke Dibridgehead Diphosphine Stators: Syntheses,Substitution Reactions,Structures, and Dynamic Properties 下载免费PDF全文
Dr. Agnieszka J. Nawara‐Hultzsch Dr. Michael Stollenz Dr. Michał Barbasiewicz Dr. Sławomir Szafert Dr. Tadeusz Lis Dr. Frank Hampel Dr. Nattamai Bhuvanesh Prof. Dr. John A. Gladysz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(16):4617-4637
Threefold intramolecular ring‐closing metatheses of trans‐[MCl2(P{(CH2)mCH?CH2}3)2] are effected with Grubbs’ catalyst. Following hydrogenation catalyzed by [RhCl(PPh3)3], the title complexes trans‐[MCl2(P((CH2)n)3P )] (n=2m+2; M/n=Pt/14, 4 c ; Pt/16, 4 d ; Pt/18, 4 e ; Pd/14, 5 c ; Pd/18, 5 e ) and sometimes isomers partly derived from intraligand metathesis, trans‐[MCl2{P(CH2)n(CH2)n}P (CH2)n)] ( 4′c–e , 5′e ), are isolated. These react with LiBr, NaI, and KCN to give the corresponding MBr2, MI2, and M(CN)2 species (58–99 %). 13C NMR data show that the MX2 moieties rapidly rotate within the diphosphine cage on the NMR timescale, even at ?120 °C. The reaction of 4 c and KSCN gives separable Pt(NCS)2 and Pt(NCS)(SCN) adducts ( 13 c , 28 %; 14 c , 20 %), and those of 4 c , e and Ph2Zn give PtPh2 species ( 15 c , 61 %; 15 e , 90 %). 13C NMR spectra of 13 c – 15 c show two sets of CH2 signals (ca. 2:1 intensity ratios), indicating that MX2 rotation is no longer rapid. Reactions of 4 c or 4′c and excess NaC?CH afford the free diphosphines P{(CH2)14}3P (91 %) and (CH2)14P (CH2)14P(C H2)14 (90 %). The latter has been crystallographically characterized as a bis(BH3) adduct. The crystal structures of eight complexes with P(CH2)14P linkages (PtCl2, PtBr2, PtI2, Pt(NCS)2, PtPh2, PdCl2, PdBr2, PdI2) and 15 e have been determined, and intramolecular distances analyzed with respect to MX2 rotation. The conformations of the (CH2)14 moieties and features of the crystal lattices are also discussed. 相似文献
60.
Maciej Jan Rybicki Anna Klimek-Turek Tadeusz Henryk Dzido 《Molecules (Basel, Switzerland)》2020,25(24)
Coccidiostats are drugs used against coccidiosis, a common disease among breeding animals. Their widespread application leads to the appearance of their residues in food, which is potentially harmful for human health and life. The European Union has established limits of concentrations of these drugs in premixtures and food. Nowadays, there are many methods for monitoring coccidiostats’ presence in market products, but their frequent weakness is sample preparation. Solvent Front Position Extraction is a planar chromatography-based sample preparation method that allows for effective assay of samples with coccidiostats when coupled with LC-MS/MS. The purpose of this research was to find common conditions for the effective isolation of eight coccidiostats from biological sample components with both lower and higher retention than the substances of interest. The acquired results were used for effective isolation of monensin and salinomycin from the premixture samples and allowed for their quantitative determination. The application of a semi-automatic device for the development of chromatograms positively impacted the results, confirming the effectiveness of the method for determining coccidiostats in biological samples. 相似文献