全文获取类型
收费全文 | 10434篇 |
免费 | 329篇 |
国内免费 | 2篇 |
专业分类
化学 | 5428篇 |
晶体学 | 132篇 |
力学 | 186篇 |
数学 | 2069篇 |
物理学 | 2950篇 |
出版年
2021年 | 148篇 |
2020年 | 140篇 |
2019年 | 128篇 |
2018年 | 199篇 |
2017年 | 222篇 |
2016年 | 467篇 |
2015年 | 308篇 |
2014年 | 353篇 |
2013年 | 801篇 |
2012年 | 485篇 |
2011年 | 621篇 |
2010年 | 400篇 |
2009年 | 342篇 |
2008年 | 319篇 |
2007年 | 273篇 |
2006年 | 203篇 |
2005年 | 161篇 |
2004年 | 156篇 |
2003年 | 157篇 |
2002年 | 167篇 |
2001年 | 137篇 |
2000年 | 128篇 |
1999年 | 138篇 |
1998年 | 129篇 |
1997年 | 126篇 |
1996年 | 134篇 |
1995年 | 118篇 |
1994年 | 112篇 |
1993年 | 123篇 |
1992年 | 116篇 |
1991年 | 104篇 |
1990年 | 105篇 |
1989年 | 101篇 |
1988年 | 100篇 |
1987年 | 122篇 |
1986年 | 88篇 |
1985年 | 128篇 |
1984年 | 129篇 |
1983年 | 122篇 |
1982年 | 118篇 |
1981年 | 114篇 |
1980年 | 116篇 |
1979年 | 125篇 |
1978年 | 119篇 |
1977年 | 113篇 |
1976年 | 112篇 |
1975年 | 98篇 |
1974年 | 103篇 |
1973年 | 102篇 |
1966年 | 82篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
171.
Titanium - catalyzed dimerization of cycloheptatriene affords pentacyclo[7.5.0.02,8.05,14.07,11]tetradeca-3,12-diene, , and pentacyclo[8.4.0.03,7.04,14.06,11]tetradeca-8,12-diene, . By heating, is converted quantitatively into . 相似文献
172.
M. Harangozó V. Frank J. Tölgyessy M. Ondráček 《Journal of Radioanalytical and Nuclear Chemistry》1994,182(1):57-62
Radionuclide X-ray fluorescence method with a Si/Li semiconductor detector and238Pn exciting source was used in the study of Mn, Fe, Cu, Zn, and Pb content of solid emissions, raw and final materials of a brick factory. From the point of view of metal content, the working environment of the brick factory is safe for workers. 相似文献
173.
H. Langfelderová L'. Macášková K. Otrubová J. Gažo 《Journal of Thermal Analysis and Calorimetry》1986,31(5):1143-1151
The thermal decompositions of Cu(en)2(NCS)X. where X–=Cl–, Br–, NO
3
–
, BF
4
–
and ClO
4
–
, have been studied in comparison with the courses of Cu(en)2(SCN)2 and Cu(en)2X2 decomposition. It is shown that the presence of the thiocyanate group in the complexes Cu(en)2(NCS)X is the most important factor in the decomposition course, in agreement with the fact that the anions X– are not coordinated. or are only semicoordinated. Significant differences were found in the courses of thermal decomposition of two forms of Cu(en)2(NCS)(BF4) differing in the structure of their coordination polyhedra.
Zusammenfassung Die thermische Zersetzung von Cu(en)2(NCS)X (X=Cl Br, NO3, BF4 und ClO4) wurde untersucht und mit der von Cu(en2(SCN)2 und Cu(en)2X2 verglichem. Der den Verlauf der Zersetzung entscheidend bestimmende Faktor isi die Anw esenheit der Thiocyanat-Gruppe in den Cu(en)2(NCS)X-Komplexen, was in Übereinstimmung damit steht,. daß die Anionen X nicht oder nur teilweise koordiniert sind. Es wurden signifikante Unterschiedeim Verlauf der thermischen Zersetzung von zwei sich in der Struktur ihrer Kordinationspolyeder unter-, heidenden Formen von Cu(en)2(NCS)(BF4) gefunden.
Cu(en)2(NCS)X (X–=Cl–, Br–, NO 4 – , NO 3 – , BF4 ClO4, Cu(en)2(SCN)2 Cu(en)2(X)2. , Cu(en)2(NCS)X X Cu(en) (NCS)BF4 i .相似文献
174.
Vivekananda S Sadílek M Chen X Turecek F 《Journal of the American Society for Mass Spectrometry》2004,15(7):1055-1067
Collisional neutralization of several isomeric C(4)H(7)O(2) cations is used to generate radicals that share some structural features with transient species that are thought to be produced by radiolysis of 2-deoxyribose. The title 2-hydroxyoxolan-2-yl radical (1) undergoes nearly complete dissociation when produced by femtosecond electron transfer from thermal organic electron donors dimethyl disulfide and N,N-dimethylaniline in the gas phase. Product analysis, isotope labeling ((2)H and (18)O), and potential energy surface mapping by ab initio calculations at the G2(MP2) and B3-PMP2 levels of theory and in combination with Rice-Ramsperger-Kassel-Marcus (RRKM) kinetic calculations are used to assign the major and some minor pathways for 1 dissociations. The major (approximately 90%) pathway is initiated by cleavage of the ring C-5[bond]O bond in 1 and proceeds to form ethylene and *CH(2)COOH as main products, whereas loss of a hydrogen atom forms 4-hexenoic acid as a minor product. Loss of the OH hydrogen atom forming butyrolactone (2, approximately 9%) and cleavage of the C-3[bond]C-4 bonds (<1%) in 1 are other minor pathways. The major source of excitation in 1 is by Franck-Condon effects that cause substantial differences between the adiabatic and vertical ionization of 1 (5.40 and 6.89 eV, respectively) and vertical recombination in the precursor ion 1(+) (4.46 eV). (+)NR(+) mass spectra distinguish radical 1 from isomeric radicals 2-oxo-(1H)oxolanium (3), 1,3-dioxan-2-yl (9), and 1,3-dioxan-4-yl (10) that were generated separately from their corresponding ion precursors. 相似文献
175.
π-electron energies and bond orders of benzenoid hydrocarbons with up to five fused hexagons have been considered by the simple
Bond Orbital Resonance Theory (BORT) approach. The corresponding ground states were determined according to four BORT models.
In the first three models a diagonalisation of the Hückel-type Hamiltonian was performed in the bases of Kekulé, of Kekulé
and mono-Claus and of Kekulé and Claus resonance structures, respectively. In the fourth model a simple BORT ansatz was used.
According to this ansatz, the ground state is a linear combination of the positive Kekulé structures, all with equal coefficients.
It was shown that π-electron energies and bond orders obtained by these models correlate much better with the PPP energies
and bond orders than with the Hückel energies and bond orders. This indicates that a simple BORT approach is quite reliable
in predicting the more sophisticated PPP results. Concerning the relative performance of the four BORT models, the best results
were obtained with the BORT ansatz. The performance deteriorates with the expansion of the basis set. This is attributed to
the fact that in these models the improvement of the basis set is not accompanied with the corresponding improvement of the
Hamiltonian. Comparing the BORT-ansatz bond orders with the Pauling bond orders, it was shown that BORT-ansatz bond orders
correlate much better with the PPP bond orders.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
176.
Twenty laboratories from 17 countries around the world participated in two analytical quality control exercises, coded NAT-5
and NAT-6, on determining trace and minor elements in plant bioindicator samples. 1398 laboratory mean values for 51 elements
were submitted for two lichen and two moss materials. The submitted measurement results were evaluated as interlaboratory
comparison (IC) exercises and as proficiency tests (PT) following standard procedures developed by the International Atomic
Energy Agency (IAEA). The evaluations confirmed good performance of the participating laboratories for many elements. Furthermore,
the laboratories performance based on IC criteria improved from the first exercise to the second one. Subsequent NAT-5 evaluation
revealed systematic differences between the analytical values obtained non-destructively or after the total sample dissolution
and the measurements following nitric acid sample dissolution (without the use of hydrofluoric acid) for some elements. The
most critical elements for this kind of discrepancies appeared to be Al, Ca, Cr, Fe, Na, Ni, and Pb. After changing analytical
methodologies in the concerned laboratories, more consistent values for those elements were obtained in NAT-6.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
177.
Syntheses and spectral characteristics of cadmium(II) compounds (CdSeO4, CdSeO3, and Cd(NCSe)2(nia)2) containing selenium in oxidation states (VI), (IV), and (-II) are described. In Cd(NCSe)2(nia)2, nicotinamide (nia) and selenocyanate anions are bonded to Cd atom as N-donor monodentate ligands. Nicotinamide is coordinated
through the ring nitrogen atom. The effects of these selenium compounds as well as Cd(NCS)2(nia)2 on the growth and Cd accumulation in roots and shoots of hydroponically cultivated chamomile plants (cultivar Lutea) were studied. In the applied concentration range (12–60 μmol dm−3) Cd(NCS)2(nia)2 affected neither the length nor the dry mass of roots and shoots. Other compounds applied at 24 μmol dm−3 and 60 μmol dm−3 significantly reduced dry mass of roots and shoots. Selenium oxidation state in the cadmium compounds affected Cd accumulation
in plant organs as well as Cd translocation within the plants, which was reflected in the values of bioaccumulation (BAF)
and translocation factors (S/R). Cd amount accumulated by shoots was lower than that in the roots. The highest BAF values
determined for Cd accumulation in shoots were obtained with CdSeO4. Substitution of S with Se in the Cd(NCX)2(nia)2 (X = Se or S) caused an increase of Cd translocation into the shoots.
Presented at the XVIIIth Slovak Spectroscopic Conference, Spišská Nová Ves, 15–18 October 2006. 相似文献
178.
František Kvasnička Jaroslav Dobiáš Kamila Klaudisová-Chudáčková 《Central European Journal of Chemistry》2003,1(1):91-97
A simple, rapid and reproducible capillary isotachophoretic on-line coupled with capillary zone electrophoresis (CITP-CZE)
method for the determination of IMz in food packaging extracts and its residues in apples is described. A good separation
of the IMZ from other sample constituents was achieved within 15 minutes without any sample clean up. Method characteristics
(linearity, accuracy, intra-assay and detection limit) were determined. Less amount of time involved, sufficient sensitivity
and low running cost are the important attributes of CITP-CZE method. 相似文献
179.
Nataša Pejić Slobodan Anić Vesna Kuntić Vladana Vukojević Ljiljana Kolar-Anić 《Mikrochimica acta》2003,143(4):261-267
A kinetic method is described for the microquantitative (microconcentration/microvolume) determination of rutin based on potentiometric monitoring of the concentration perturbations of the Bray-Liebhafsky (BL) oscillatory reaction being in a non-equilibrium stationary state close to a bifurcation point. The experiments are carried out in an open reactor. The response of the matrix system to perturbations by different concentrations of rutin ethanolic solutions is followed by a Pt-electrode. In the concentration range between 7.8×10–8moldm–3 and 9.1×10–6mol dm–3, we found a linear dependence of the maximal potential shift, Em, on the logarithm of the rutin concentrations. The unknown concentrations can be determined from the calibration curve up to an accuracy of ±5%. The detection limit is 3.6×10–8mol dm–3. The amount of required sample can be as small as 10µL. 相似文献
180.
P. Rajec F. Macášek M. Féder P. Misaelides E. Šamajová 《Journal of Radioanalytical and Nuclear Chemistry》1998,229(1-2):49-55
The sorption of caesium and strontium from its aqueous solutions by ten clinoptilolite-and mordenite-containing sedimentary
materials from Slovakia, Bulgaria, Ukraine and Greece was investigated by batch-type procedure and radiotracer techniques.
The concentration of the solutions, that were performed in and without the presence of competing cations (0.005 mol·dm−3 KCl) varied between 1·10−4 and 5·10−2 mol·dm−3. The uptake and distribution coefficient (K
d
) values determined for the materials of the different origin, were correlated with their mineralogical composition and gross
cation exchange capacity (CEC). The identification of the specific uptake sites was attempted on the basis of the sorption
isotherms and the content of exchangeable cations. The experimental results provide information on the suitability of the
individual materials for the treatment of radioactive wastes and their application as backfills in potential nuclear waste
repositories. 相似文献