全文获取类型
收费全文 | 1285篇 |
免费 | 54篇 |
国内免费 | 12篇 |
专业分类
化学 | 872篇 |
晶体学 | 11篇 |
力学 | 31篇 |
数学 | 185篇 |
物理学 | 252篇 |
出版年
2023年 | 3篇 |
2022年 | 14篇 |
2021年 | 25篇 |
2020年 | 21篇 |
2019年 | 21篇 |
2018年 | 19篇 |
2017年 | 23篇 |
2016年 | 35篇 |
2015年 | 26篇 |
2014年 | 41篇 |
2013年 | 75篇 |
2012年 | 76篇 |
2011年 | 107篇 |
2010年 | 65篇 |
2009年 | 43篇 |
2008年 | 90篇 |
2007年 | 103篇 |
2006年 | 81篇 |
2005年 | 56篇 |
2004年 | 61篇 |
2003年 | 42篇 |
2002年 | 41篇 |
2001年 | 14篇 |
2000年 | 18篇 |
1999年 | 19篇 |
1998年 | 17篇 |
1997年 | 15篇 |
1996年 | 15篇 |
1995年 | 13篇 |
1994年 | 13篇 |
1993年 | 17篇 |
1992年 | 12篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1975年 | 7篇 |
1974年 | 4篇 |
1973年 | 6篇 |
排序方式: 共有1351条查询结果,搜索用时 15 毫秒
21.
Uyguner CS Hellriegel C Otto W Larive CK 《Analytical and bioanalytical chemistry》2004,378(6):1579-1586
Humic substances are precursors of carcinogenic trihalomethanes (THMs) formed during disinfection by chlorination in water treatment processes. In an effort to understand the relationship between trihalomethane formation potential (THMFP) and physicochemical properties of humic substances, UV-visible absorbance, fluorescence in emission and synchronous scan modes, and NMR spectra were measured for several aquatic fulvic and humic acids. For comparison, a soil fulvic acid was also examined using these methods. The feasibility of the gradient modified spin-echo (GOSE) NMR experiment to selectively measure singlet resonances arising from isolated protons was examined. In addition, diffusion coefficients were measured for DMSO solutions of the fulvic acids using BPPLED and GOSE-edited pulse sequences. Although none of the methods tested produced results that correlated with THMFP, the GOSE intensities determined for different regions of the NMR spectra did reflect the relative abundance of different types of functional groups produced by lignin oxidation. In addition, the GOSE-edited diffusion results suggest that the isolated protons, those most reactive to chlorination, are more likely contained in the larger molecular weight fractions of fulvic acids. 相似文献
22.
Biener MM Biener J Friend CM 《Langmuir : the ACS journal of surfaces and colloids》2005,21(5):1668-1671
The Au-S interaction is probably the most intensively studied interaction of Au surfaces with nonmetals, as, for example, it plays an important role in Au ore formation(1) and controls the structure and dynamics of thiol-based self-assembled monolayers (SAMs). Various S-induced surface structures on Au(111) were recently reported for different conditions and predominantly interpreted in terms of a static Au surface. Here, we demonstrate that the Au(111) surface exhibits a very dynamic character upon interaction with adsorbed sulfur: large-scale surface restructuring and incorporation of Au atoms into a growing 2D AuS phase were observed in situ. These results provide new insight into the Au-S surface chemistry. 相似文献
23.
Rezende MC Flores P Guerrero J Villarroel L 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(7):1637-1640
The 13C NMR of two solvatochromic dyes derived from a barbituric acid acceptor and dimethylaminophenyl donor fragments, compound 1 and the related merocyanine 2, were recorded in various solvents. The observed chemical-shift variations were used to interpret their structural differences and solvatochromic behavior in solution. 相似文献
24.
Horacio R. Flores 《Thermochimica Acta》2007,452(1):49-52
Ulexite has a substantial amount of hydration water, which allows using calcination methods to increase the B2O3 content from 42.97 to 66.69%, due to the loss of 35.54% of its weight, corresponding to its hydration water.In this study, determinations of changes in the heat content at each temperature were made using an isoperibol calorimeter. The mean specific heat was calculated from these values. The reaction enthalpy was also determined by DTA methods.The sample used in the experiments contains 39.04% B2O3. The temperature was between 80 and 1000 °C. Curves of weight loss and enthalpy changes are presented and analysed.The results have industrial application in order to determine the thermal requirements for calcining ulexite at different temperatures, as well as its complete dehydration. 相似文献
25.
John Philip Tarburton Cynthia S. Day Victor W. Day Iraj Tavaniepour Norman H. Cromwell 《Journal of heterocyclic chemistry》1986,23(2):433-441
Solid-state structures have been determined for cis- and trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines using single-crystal X-ray diffraction techniques. The cis isomer crystallizes in the centrosymmetric monoclinic space group P21/c (No. 14), with a = 18.669(3)Å, b = 5.709(1)Å, c = 17.412(2)Å, β = 96.29(1)° and Z = 4; the trans isomer crystallizes in the noncentrosymmetric orthorhombic space group Pna21 (No. 33), with a = 17.089(2)Å, b = 18.729(3)Å, c = 5.749(1)Å and Z = 4. Full-matrix least-squares refinement of the structural parameters led to the following final agreement factors: R1 (unweighted, based on F) = 0.040 and R2 (weighted, based on F) = 0.054 for the 2592 independent reflections of the cis isomer having 2θMoK¯α <55° and I>3σ1, and R1 = 0.033 and R2 = 0.031 for the 1504 independent reflections of the trans isomer having 2θMoK¯α <55° and I>3σ1. The statistically significant differences that exist between the two isomers for two bond lengths and ten bond angles (p < 0.05) appear to be the direct result of the p-toluyl group orientation with respect to the cyclohexyl and phenyl substituents. In the cis isomer it is anti with respect to the N-cyclohexyl group and cis with respect to the phenyl group, whereas in the trans isomer it is syn with respect to the N-cyclohexyl and trans with respect to the phenyl group. Three-ring to carbonyl hyperconjugation is correlated with stereoelectronic interactions in the trans isomer. Bonding, determined by X-ray and nmr studies, is discussed for the three-membered aziridine ring proper; while bonding, determined by X-ray studies, is discussed for substituents of the aziridine ring. These aziridinyl ketone compounds are of importance as potential mammalian DNA alkylating anti-tumor agents in solid-state solid-state systems. To date only a trans isomer has demonstrated this biological activity in tumor-bearing rats. 相似文献
26.
Linda L LipeSuzanne M Purinton Elizabeth MederiosC.Chad Harrell Cynthia EftaMelissa Murray Meredith WoodRay-Bernard Portier Stuart J Chalk 《Analytica chimica acta》2002,455(2):287-304
A new apparatus for mixing sample and reagent in flow injection analysis (FIA) is described. The continuously variable volume reactor (CVVR) replaces the conventional mixing coil in a flow injection (FI) manifold to provide mixing and dilution. A linear actuator motor allows control of the chamber volume via LabVIEW software. The chamber volume can be incremented in steps of 1 μl over the range 68-1704 μl. In addition, the chamber has an integral variable-speed stirring unit that is also under computer control. Experiments were performed to evaluate the dispersion characteristics of this new device, evaluate the volume reproducibility, and understand the mixing characteristics. Use of the chamber is shown in the determination of iron(II) in pond water, and in NIST SRM 1643d with excellent results and a detection limit of 3.7 μg/l iron(II). Advantages of the CVVR and future research activities using the device are discussed. 相似文献
27.
William H. McCarroll Cynthia Darling Gary Jakubicki 《Journal of solid state chemistry》1983,48(2):189-195
Fused salt electrolysis has been used to prepare a number of reduced oxides of molybdenum with lanthanum, neodymium, and yttrium in single crystal or oriented polycrystalline form. The average valence of molybdenum in the various compounds ranged from 5.67 to 3.50. Previously unreported compounds include La5Mo4O16 (triclinica = 5.64 Å,b = 20.7 0Å,c = 5.64 Å, α = 86.55°, β = 90.0°, γ = 93.45°); La2Mo2O7 (orthorhombic,a = 12.19 Å,b = 6.05 Å,c = 3.87 Å); LaMo2O5 (hexagonal,a = 8.378 Å,c = 19.26 Å). In addition, single crystal specimens have been prepared of Y2MoO5,Ln5Mo3O16(Ln =La, Nd) and metal atom cluster compounds of theA2Mo3O8 type (A = Mg, Co, Ni, Zn). 相似文献
28.
Joseph H. Montoya Kirsten T. Winther Raul A. Flores Thomas Bligaard Jens S. Hummelshj Muratahan Aykol 《Chemical science》2020,11(32):8517
We present an end-to-end computational system for autonomous materials discovery. The system aims for cost-effective optimization in large, high-dimensional search spaces of materials by adopting a sequential, agent-based approach to deciding which experiments to carry out. In choosing next experiments, agents can make use of past knowledge, surrogate models, logic, thermodynamic or other physical constructs, heuristic rules, and different exploration–exploitation strategies. We show a series of examples for (i) how the discovery campaigns for finding materials satisfying a relative stability objective can be simulated to design new agents, and (ii) how those agents can be deployed in real discovery campaigns to control experiments run externally, such as the cloud-based density functional theory simulations in this work. In a sample set of 16 campaigns covering a range of binary and ternary chemistries including metal oxides, phosphides, sulfides and alloys, this autonomous platform found 383 new stable or nearly stable materials with no intervention by the researchers.We present an end-to-end computational system for autonomous materials discovery. 相似文献
29.
J. Rodríguez Flores J. J. Berzas Nevado G. Castañeda Peñalvo M. I. Rodríguez Cáceres 《Chromatographia》2003,57(7-8):493-496
Summary Capillary zone electrophoresis (CZE) has been used for direct determination of 6-thioguanine, methotrexate, and 5-fluorouracil
in human urine, by use of a fused-silica capillary (60.2 cm×75 μm i.d.). Separation was performed after hydrodynamic injection
for 7 s; the separation potential and capillary temperature were 25 kV and 35°C, respectively. A 45mm borate buffer solution (pH 9.2) was used as separation electrolyte. Under these conditions the analysis takes approximately
10 min and interday precision of migration times and corrected peak areas is satisfactory. A linear response over the concentration
range 3.0–20.0 mg L1 was observed for the three chemotherapeutic drugs in diluted human urine. Detection limits (s/n=3) for 6-thioguanine and methotrexate were approximately 1.60 mg L1 in diluted human urine; that for fluorouracil was 2.60 mg L1. A 2-ml volume of human urine was diluted with 2-mL of water and introduced directly into the electrophoresis system. CZE
was shown to be a good method with regard to simplicity, satisfactory precision, and sensitivity. This method resulted in
especially excellent recoveries for determination of methotrexate in all the different urine samples analysed (n=10). 相似文献
30.
Araceli Vernica Flores Carlos Alberto Prez Marco Aurlio Zezzi Arruda 《Analytica chimica acta》2005,530(2):619-305
In the present work, scandium was determined in sediment slurry samples (from three different rivers) by electrothermal atomic absorption spectrometry (ETAAS). Slurries were prepared by weighting 100 mg of dry sediment samples (≤53 μm particle sizes) and adding 6 ml of HCl:HNO3:HF (3:1:2, v/v). Accurate results were only possible due to the synergetic effect between Rh as permanent chemical modifier and acetylacetone (Acac) as complexing agent. The same platform was used for 400 heating cycles. The performance of the chemical modification was evaluated by using scanning electron microscopy (SEM), synchrotron radiation X-ray fluorescence (SRXRF) and some figures of merit (precision and detectability). The best analytical conditions were attained using 1500 and 2550 °C as pyrolysis and atomization temperatures. The scandium content in the liquid phase of the slurries ranged from 61 to 73%, thus indicating, in this study, that both liquid and solid phases play an important role in slurry analyses. An amount of 5.0–20.0 μg l−1 Sc linear range as well as LOD and LOQ of 0.19 and 0.62 μg l−1, respectively, were obtained under these conditions. The accuracy was checked by using microwave-assisted decomposition, and the results compared to those obtained with the proposed methodology (slurry analysis). By checking both sets of the results, there is no statistical difference at the 95% confidence levels. 相似文献