全文获取类型
收费全文 | 84553篇 |
免费 | 643篇 |
国内免费 | 489篇 |
专业分类
化学 | 29240篇 |
晶体学 | 826篇 |
力学 | 7032篇 |
数学 | 32599篇 |
物理学 | 15988篇 |
出版年
2023年 | 60篇 |
2022年 | 284篇 |
2021年 | 369篇 |
2020年 | 379篇 |
2019年 | 383篇 |
2018年 | 10659篇 |
2017年 | 10460篇 |
2016年 | 6455篇 |
2015年 | 1175篇 |
2014年 | 752篇 |
2013年 | 1165篇 |
2012年 | 4362篇 |
2011年 | 10954篇 |
2010年 | 5930篇 |
2009年 | 6334篇 |
2008年 | 6880篇 |
2007年 | 8998篇 |
2006年 | 405篇 |
2005年 | 1454篇 |
2004年 | 1625篇 |
2003年 | 2069篇 |
2002年 | 1092篇 |
2001年 | 271篇 |
2000年 | 301篇 |
1999年 | 176篇 |
1998年 | 207篇 |
1997年 | 163篇 |
1996年 | 217篇 |
1995年 | 124篇 |
1994年 | 88篇 |
1993年 | 102篇 |
1992年 | 66篇 |
1991年 | 74篇 |
1990年 | 58篇 |
1989年 | 67篇 |
1988年 | 63篇 |
1987年 | 71篇 |
1986年 | 62篇 |
1985年 | 58篇 |
1984年 | 55篇 |
1982年 | 55篇 |
1981年 | 46篇 |
1980年 | 60篇 |
1979年 | 52篇 |
1978年 | 43篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The heterogeneously catalyzed liquid phase Fries rearrangement reaction of phenyl acetate was carried out on Beta zeolites reexchanged with different amounts of metal cations. The aim of this work was to determine the influence of the amount of Brønsted acid sites as the catalytic active centers on the conversion. Sodium and potassium ions as well as divalent calcium and zinc ions were used in the reexchange procedure. The conversion shows a linear dependency on the degree of ion reexchange following the theoretically expected values. So the results prove that the bridged hydroxyl groups are indeed the dominating active centers and that their strength seems to be independent of the degree of reexchange; consequently, their acidic strength should be identical. A special emphasis was placed on the reexchange with divalent metal ions. It was shown that one divalent ion is not able to replace two protons as is supposed to be necessary for the charge balance—so a 1:1 stoichiometry is needed to assume to explain the catalytic results. 相似文献
992.
The cytochrome P450 (P450) enzymes are mainly localized to the endoplasmic reticulum (ER), where they function within catalytic
complexes metabolizing xenobiotics and some endogenous substrates. However, certain members of families 1–3 were also found
in other subcellular compartments, such as mitochondria, plasma membrane, and lysosomes. The physiological function of these
enzymes in non-ER locations is not known, although plasma-membrane-associated P450s have been described to be catalytically
active and to participate in immune-mediated reactions with autoantibody formation that can trigger drug-induced hepatitis.
Several retention/retrieval mechanisms are active in the ER retention of the P450s and inverse integration of the translated
P450 into the ER membrane appears to be responsible for transport to the plasma membrane. Furthermore, hydrophilic motifs
in the NH2-terminal part have been suggested to be important for mitochondrial import. Phosphorylation of P450s has been described to
be important for increased rate of degradation as well as for targeting into mitochondria. It was also suggested that the
mitochondria-targeted P450s from families 1–3 could be active in drug metabolism using an alternative electron transport chain.
In this review we present an update of the field emphasizing studies concerning localization, posttranslational modification,
such as phosphorylation, and intracellular transport of microsomal P450s. 相似文献
993.
Inis Jenemann Susanne Kiefer E. Robens 《Journal of Thermal Analysis and Calorimetry》2008,94(3):607-612
The short survey covers the development of the balance since its invention in the Neolithic era. Scales have been used most
probably first as tools in trading, but already in Old Egyptian papyrus its use in techniques is documented. Its theory was
cleared by Aristotle and Archimedes and at least at that time it was used as a scientific instrument. Today the balance is
still the most widely used instrument in science and there are still improvements. 相似文献
994.
Using Stuttgart/Dresden effective core potentials MWB28, MWB60, and GTO valence basis sets (8s7p6d)/[6s5p3d], (8s7p6d)/[6s5p3d]
for Zr and Hf atoms and 6-311++G(3df,3pd) basis set for C, H, O, and Ti atoms, tight convergence criteria geometry optimizations
and harmonic frequency calculations are performed at B3LYP and B3LYP/IEF-PCM levels of theory so as to model the gas phase
and argon matrix infrared spectra of the tetrahedral molecules R2MH2 (R = D(H), CH3, OH; M = Ti, Zr, Hf). Influence of the transition metal and/or substituent group on the symmetric and asymmetric stretching
frequencies of the MH2 fragment of the R2MH2 molecules is investigated at both the levels of theory. The modelling of the argon matrix effect improves the agreement between
the calculated frequencies and the experimental ones. The calculated argon matrix to gas phase frequency shifts is compared
reasonably to the experimental argon to neon matrix shifts. 相似文献
995.
A. Varesano C. Tonin F. Ferrero Marinella Stringhetta 《Journal of Thermal Analysis and Calorimetry》2008,94(2):559-565
Electro-conducting doped polypyrrole was deposited by in situ oxidative polymerisation on PET non-wovens. Thermal properties were evaluated by means of DSC and TG in nitrogen and air.
Flame resistance tests reveal that coated PET fibres resist to direct contact with fire. By thermal analysis, it was found
that PPy reduces the temperature at which thermo-oxidative degradation of PET occurs. Polypyrrole-coated PET non-wovens were
heated above the melting point of PET for 30 min. After the heating the fibres become brittle and frail, but SEM observations
revealed that they maintained their fibrous shape. A loss of chlorine was found because of intense heating. 相似文献
996.
The molecular geometries and vibrational frequencies of aluminum oxyhalides, AlOX, and their dimers, (AlOX)2, have been calculated by MP2 and density functional methods. The monomeric molecules are linear while the dimers have a D
2h
-symmetry geometry with a four-membered ring, in which the oxygen atoms serve as bridging ligands. Different thermodynamic
properties have been calculated; for AlOBr and AlOI for the first time. 相似文献
997.
3,5-Bis(2-hydroxyphenyl)-1H-1,2,4-triazole (H2La) and 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H3Lb) have been prepared, and crystal structure of the intermediate 2-(2-hydroxyphenyl)-4H-1,3-benzoxazin-4-one has been determined. Temperature dependent 1H NMR spectroscopic measurements of H2La indicated dynamic behavior with the equilibrium between the two asymmetric tautomers. For H3Lb, pD-dependent 1H NMR spectroscopic measurements showed small but characteristic shifts in the range of 0 ≤ pD ≤ 1, indicative of a triazole
nitrogen atom protonation; the corresponding pK
a of 0.98 ± 0.04 was determined by spectrophotometric titrations. (H2O, 26°C, 1 M KCl/HCl). Formation of [FeIII(La)]+ (pH 2.5) and [FeIII(La)2]− (pH > 6) was verified by UV-Vis spectroscopy. Complex formation of H3Lb with Al3+ and VO2+ was investigated by 1H NMR spectroscopic titration and cyclic voltammetry, respectively. Single crystals of the phenoxo bridged [VVO(HLb)(EtO)]2·2EtOH were characterized by X-ray structural analysis.
Dedicated to Professor Milan Melník on the occasion of his 70th birthday 相似文献
998.
Methodologies for the quantification of measurement uncertainties associated with the determination of 210Pb- and 210Po-specific activities by liquid scintillation counting (LSC) and alpha-particle spectrometry are presented, and are demonstrated
using the soil reference material IAEA-326. Major contributors to the combined uncertainty associated with the measurement
result of 210Pb were the uncertainties of net count rates in the 210Pb energy region of the sample spectrum and in the 210Bi energy region of the blank spectrum. The predominant sources of uncertainty in the measurement of 210Po were the uncertainties of net count rates in the regions of interest of 209Po and 210Po. The relative standard uncertainty of 210Po exponentially increases with the time interval between the sampling date and the separation date of Po, and this effect
is strongly dependent on the 210Po/210Pb activity ratio. When the specific activity of 210Pb is much higher than that of 210Po in the sample, the relative standard uncertainty of the 210Po determination increases significantly within a short time interval between the sampling date (or reference date) and the
separation date of Po in samples. 相似文献
999.
In this paper, we introduce and study a model of a Monod–Haldene type food chain chemostat with pulsed substrate. We investigate
the subsystem with substrate and prey and study the stability of the periodic solutions, which are the boundary periodic solutions
of the system. The stability analysis of the boundary periodic solution yields an invasion threshold. By use of standard techniques
of bifurcation theory, we prove that above this threshold there are periodic oscillations in substrate, prey, and predator.
Simple cycles may give way to chaos in a cascade of period-doubling bifurcations. Furthermore, by comparing bifurcation diagrams
with different bifurcation parameters, we can see that the impulsive system shows two kinds of bifurcations, whose are period-doubling
and period-halfing. 相似文献
1000.
Sephadex LH-20 column chromatography was used to separate flavonoid components in a heartsease methanol extract. One of the
main components was identified by NMR as violanthin (6-C-glucosyl-8-C-rhamnosylapigenin). As a first approximation, the other main flavonoid component was considered to be rutin (3-O-rhamnoglucosylquercetin), based on comprehensive comparison of retention times and UV spectra of reference molecules, as
well as molecular mass and fragmentation patterns obtained by mass spectrometry. The minor flavonoids were separated by polyamide
column and analyzed by LC-MS. The antioxidant capacity of different flavonoid fractions was determined using both Trolox equivalent
antioxidant capacity (TEAC) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) in vitro antioxidant assays. The highest electron-donor
capacity was found for the major flavonoid component (rutin), whereas one minor component-rich flavonoid fraction exhibited
the highest hydrogen-donor activity. 相似文献