全文获取类型
收费全文 | 5080篇 |
免费 | 184篇 |
国内免费 | 24篇 |
专业分类
化学 | 3603篇 |
晶体学 | 42篇 |
力学 | 110篇 |
数学 | 711篇 |
物理学 | 822篇 |
出版年
2024年 | 4篇 |
2023年 | 34篇 |
2022年 | 87篇 |
2021年 | 119篇 |
2020年 | 85篇 |
2019年 | 81篇 |
2018年 | 70篇 |
2017年 | 74篇 |
2016年 | 184篇 |
2015年 | 128篇 |
2014年 | 152篇 |
2013年 | 301篇 |
2012年 | 373篇 |
2011年 | 410篇 |
2010年 | 222篇 |
2009年 | 182篇 |
2008年 | 349篇 |
2007年 | 323篇 |
2006年 | 277篇 |
2005年 | 314篇 |
2004年 | 237篇 |
2003年 | 213篇 |
2002年 | 186篇 |
2001年 | 73篇 |
2000年 | 70篇 |
1999年 | 34篇 |
1998年 | 57篇 |
1997年 | 36篇 |
1996年 | 71篇 |
1995年 | 35篇 |
1994年 | 38篇 |
1993年 | 51篇 |
1992年 | 29篇 |
1991年 | 33篇 |
1990年 | 41篇 |
1989年 | 29篇 |
1988年 | 30篇 |
1987年 | 35篇 |
1986年 | 22篇 |
1985年 | 35篇 |
1984年 | 32篇 |
1983年 | 15篇 |
1982年 | 21篇 |
1981年 | 17篇 |
1980年 | 13篇 |
1979年 | 15篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1974年 | 7篇 |
1973年 | 5篇 |
排序方式: 共有5288条查询结果,搜索用时 515 毫秒
81.
The design and construction of a simple device for measuring ionic concentrations (or pH) with ion-selective electrodes are described. The automated system includes a special electronic circuit with an operational amplifier, a signal conditioner and a personalcomputer. A digital multimeter can be used if automation is not required. The results obtained in tests with iodide-, chloride- and nitrate-selective electrodes and glass electrodes show very good agreement with those obtained with sophisticated commercial apparatus. 相似文献
82.
A chemometric study on the prediction of the main nutritional aspects of milk has been carried out by using fourier transform infrared spectroscopy (FTIR) attenuated total reflectance (ATR) measurements of commercially available milk samples of different types. Whole, semi and skimmed milks, enriched or not with calcium, vitamins or modified by alteration of lipid or sugar composition were considered. After evaluating different strategies for data acquisition and ATR cleaning between samples, hierarchical cluster analysis (HCA) was carried out for classification of samples in order to choose the calibration set. The prediction capabilities of partial least squares (PLS) data treatment were evaluated in order to obtain information about total fat, total protein, total carbohydrates (CH), calories and calcium. On using the mean square error of cross-validation and prediction as control variables, a critical evaluation were made about the calibration set to be used, the spectral range to be considered and the data treatment (PLS-1 or PLS-2) to be performed. By selecting a calibration set of 33 samples the properties of 48 samples were predicted with relative precision of triplicates of 0.062, 0.040 and 0.039% w/v for total fat, protein and carbohydrates, and 0.66 kcal/100 ml for calories, and 2.1 mg of Ca/100 ml. The mean difference (dx−y) between predicted and actual values and standard deviation of mean differences (sx−y), were of 0.06 (0.38), 0.03 (0.18) and −0.15 (0.41), being sx−y values between brackets, for total fat, proteins and carbohydrates, 0.06 (3.8) kcal/100 ml for calories and −4.5 (9) mg/100 ml for calcium.The sensitivity and selectivity of the methodology developed were evaluated on terms of the net analyte signal. Selectivity factors ranging from 2 to 7.6% have been calculated for the five parameters considered. 相似文献
83.
Pérez J Morales D Nieto S Riera L Riera V Miguel D 《Dalton transactions (Cambridge, England : 2003)》2005,(5):884-888
Cationic complexes [Mo(eta(3)-allyl)(CO)2L3]+ (L3 = either nitrogen-donor tridentate ligand or three monodentate ligands) were prepared in high yield and under mild conditions using as precursors either the triflato complex [Mo(eta(3)-allyl)(OTf)(CO)2(NCMe)2] or the combination of the chloro complex [Mo(eta(3)-allyl)Cl(CO)2(NCMe)2] and the salt NaBAr'(4)(Ar'= 3,5-bis(trifluoromethyl)phenyl). The tridentate ligands employed were 2,2':6',2'-terpyridine (terpy) and cis,cis-1,3,5-cyclohexanetriamine (CHTA), whereas the monodentate ligands imidazole (im) and 3,5-dimethylpyrazole (dmpz) were chosen. In order to stabilize the labile intermediates, an excess of acetonitrile was used in most of the syntheses. However, the pyrazole complex was prepared through a nitrile-free route to avoid reactions at the coordinated nitrile. The solid state structures of [Mo(eta(3)-methallyl)(CO)2(terpy)]OTf (2), [Mo(eta(3)-methallyl)(CO)2(CHTA)]BAr'4 (3), [Mo(eta(3)-methallyl)(CO)2(NCMe)3]BAr'4 (4), [Mo(eta(3)-allyl)(CO)2(im)3]OTf (5) and [Mo(eta(3)-allyl)(CO)2(dmpz)3]BAr'4 (6) were determined by means of single-crystal X-ray diffraction. 相似文献
84.
Simple and rapid micellar electrokinetic capillary chromatographic method for simultaneous determination of four antiepileptics in human serum 总被引:1,自引:0,他引:1
A very rapid and simple MEKC method was developed for the simultaneous determination of four antiepileptic drugs, ethosuximide (Etho), primidone (Pri), phenytoin (Pht) and carbamazepine (Cbz) in human serum. Sample analysis required only 100 microL of human serum which only needed to be centrifuged, decanted and combined with the running buffer [5.3 mM Na(2)HPO(4)/3.2 mM borax buffer (pH 9.5) containing 55 mM SDS and 3.5% (v/v) acetone]. The analysis was performed in only 10 min into fused-silica capillaries (57 cm total length with 50 microm i.d. and 50 cm to the detector) using the MEKC methodology with diode-array detection at 220 nm. The calibration graphs were established for ethoximide, primidone, phenytoin and carbamazepine between 0 and 20 mg/L. Recoveries were between 85 and 87%. The simplicity of the proposed methodology makes it suitable for routine clinical use, especially for epileptic patients on polytherapy. 相似文献
85.
Ródenas-Torralba E Cava-Montesinos P Morales-Rubio A Cervera ML De La Guardia M 《Analytical and bioanalytical chemistry》2004,379(1):83-89
The aim of this study is to show the advantages of the emerging multicommutation methodology based on the use of solenoid valves for Te determination in milk by hydride generation atomic fluorescence spectrometry (HG-AFS). The delivery of a series of alternating sequential insertions of small volumes of samples and reagents gives rise to new hydrodynamic processes and exciting analytical potentials by controlling the time of flow through the on/off-switched solenoid valves. This drastically reduces the reagent consumption by a factor of 4 and the generation of effluents (590 mL h–1 instead of 750 mL h–1 generated by the continuous-mode measurement) and also provides an improvement in the laboratory productivity by an increase of the sample throughput (85 h–1 compared to 20 h–1 found in the continuous mode). So, multicommutation is an environmentally and economically sustainable alternative to the methodology based on continuous measurements. The multicommutation-based method developed was applied to tellurium determination in commercially available milk samples; a calibration range of 0.0–0.5 ng mL–1 and a detection limit of 0.20 ng L–1 with average relative standard deviation of 2.1% were found. Comparable results were obtained for a series of samples using both continuous and multicommutation HG-AFS modes. 相似文献
86.
Fuentes-Cabrera M Nicholson DM Sumpter BG Widom M 《The Journal of chemical physics》2005,123(12):124713
We investigate the possibility of tailoring the electronic properties of isoreticular metal-organic materials by replacing the metal atom in the metal-organic cluster and by doping. The electronic structure of M-IRMOF1, where IRMOF1 stands for isoreticular metal-organic framework 1 and M = Be, Mg, Ca, Zn, and Cd, was examined using density-functional theory. The results show that these materials have similar band gaps (ca. 3.5 eV) and a conduction band that is split into two bands, the lower of which has a width that varies with metal substitution. This variation prompted us to investigate whether doping with Al or Li could be used to tailor the electronic properties of the Zn-IRMOF1 and Be-IRMOF1 materials. It is shown that replacing one metal atom with Al can effectively be used to create IRMOFs with different metallic properties. On the other hand, adding Li produces structural changes that render this approach less suitable. 相似文献
87.
The surface pressure (Π) vs surface concentration (Γs) curves of the hydrogen-bonded polymer blend poly(vinylacetate)+ poly(4-hydro-xystyrene) (PVAc+P4HS) have been measured at
25 °C onto a water subphase at pH=2.0. While PVAc forms extended monolayers, and the free surface of water is found to be
a good solvent for it, P4HS forms compressed monolayers, and the surface is a near Θ-type solvent for it. PVAc and P4HS form
miscible non-ideal monolayers until near the collapse pressure through the whole concentration range. The composition dependence
of the Π–Γs curves is rather complex. Contrary to what might be expected, the addition of PVAc to the blend does not reduce the rigidity
of the monolayer until its weight fraction is larger than 0.5. The compressibility data of the P4HS-rich monolayers suggest
the existence of a second maximum at high surface coverages, a result already observed in some polysiloxanes.
Received: 11 March 1998 Accepted: 7 May 1998 相似文献
88.
Lhiaubet-Vallet V Sarabia Z Boscá F Miranda MA 《Journal of the American Chemical Society》2004,126(31):9538-9539
A remarkable stereodifferentiation has been observed in the interaction between the excited triplet state of carprofen (CP) and human serum albumin (HSA). Time-resolved measurements using laser flash photolysis reveal the presence of two components with different lifetimes in triplet decay. This is explained by complexation of CP to the two possible HSA binding sites. The shorter-lived components are ascribed to the CP/HSA complexes in site I, where stereodifferentiation is more important (tauR/tauS ca. 4). This is correlated with formation of a dehalogenated photoproduct upon steady-state photolysis. 相似文献
89.
Spectrofluorimetric characteristics of pindolol have been investigated with the aim of using this technique for analytical determinations. Other monosubstituted indole derivatives, 4-methoxy and 5-methoxyindole, have been also studied for comparative purposes. Corrected excitation and emission wavelengths in different solvents are reported and the effect of solvent on the Stokes shifts of these compounds has been analysed using the Lippert equation. In addition, the Stokes shift of pindolol has been determined in dioxan-water solvent mixtures and the presence of specific solvent effects is discussed. The fluorescence of pindolol is pH dependent, the quantum yields determined in water are lower than those in other solvents. With respect to the sensitivity, it has been found that the detection limits in aqueous solutions are improved in the presence of beta and methyl-beta-cyclodextrin. Finally, a fluorimetric analysis of the interaction between pindolol and different cyclodextrins has been carried out in order to determine the apparent stability constants of the complexes and the thermodynamic parameters associated to complexation. 相似文献
90.
Fernando Lazaro M. Dolores Luque de Castro Miguel Valcarcel 《Fresenius' Journal of Analytical Chemistry》1988,332(7):809-812
Summary A fast and inexpensive fluorimetric method for the determination of total aflatoxins (B1, B2, G1, and G2) in food of use in screening numerous samples suspectedly containing these substances is proposed. The sensitivity of the method (determination range between 0.5 and 200.0 ng ml–1) allows these analytes to be detected at concentrations well below legal limits; hence, separation-detection techniques such as HPLC need only be used with samples in which these compounds are found to occur. The method has been applied to maize, peanut and tapioca samples, obtaining average recoveries of 100.9 with deviations of ±5% with respect to 100% recovery.
Fluorometrische Bestimmung von Aflatoxinen durch Fließinjektionsanalyse相似文献