全文获取类型
收费全文 | 10279篇 |
免费 | 996篇 |
国内免费 | 429篇 |
专业分类
化学 | 3855篇 |
晶体学 | 56篇 |
力学 | 1848篇 |
综合类 | 96篇 |
数学 | 2212篇 |
物理学 | 3637篇 |
出版年
2023年 | 55篇 |
2022年 | 138篇 |
2021年 | 148篇 |
2020年 | 154篇 |
2019年 | 217篇 |
2018年 | 151篇 |
2017年 | 222篇 |
2016年 | 239篇 |
2015年 | 219篇 |
2014年 | 346篇 |
2013年 | 462篇 |
2012年 | 383篇 |
2011年 | 583篇 |
2010年 | 481篇 |
2009年 | 676篇 |
2008年 | 613篇 |
2007年 | 583篇 |
2006年 | 562篇 |
2005年 | 432篇 |
2004年 | 306篇 |
2003年 | 338篇 |
2002年 | 271篇 |
2001年 | 225篇 |
2000年 | 234篇 |
1999年 | 196篇 |
1998年 | 206篇 |
1997年 | 170篇 |
1996年 | 135篇 |
1995年 | 103篇 |
1994年 | 116篇 |
1993年 | 95篇 |
1992年 | 100篇 |
1991年 | 84篇 |
1990年 | 94篇 |
1989年 | 48篇 |
1988年 | 42篇 |
1987年 | 41篇 |
1985年 | 434篇 |
1984年 | 290篇 |
1983年 | 238篇 |
1982年 | 217篇 |
1981年 | 154篇 |
1980年 | 119篇 |
1979年 | 139篇 |
1978年 | 79篇 |
1977年 | 88篇 |
1976年 | 90篇 |
1975年 | 88篇 |
1974年 | 119篇 |
1973年 | 133篇 |
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
111.
The phase diagram for the binary system K2CrO4CaCrO4 has been determined for CaCrO4 concentrations up to 60 mole%, using the techniques of differential thermal analysis, X-ray diffraction, and drop calorimetry. Essential features of the phase diagram are: the solid-solid phase transition for pure K2CrO4 at 670°C, β-K2CrO4 ? α-K2CrO4; a eutectoid reaction at 14 mole% CaCrO4 and 548°C, β-K2CrO4 ? α-K2CrO4 + K2CrO4 · CaCrO4; a peritectoid event at 50 mole% CaCrO4 and 640°C, β-K2CrO4 + CaCrO4 ? K2CrO4 · CaCrO4; and a eutectic reaction at 51 mole% CaCrO4 and 678°C, L ? β-K2CrO4 + CaCrO4. X-ray diffraction studies lead to the determination of the unit cell dimensions for the K2CrO4 · CaCrO4 double salt, a C-centered monoclinic form with a0 = 7.615(6) Å, b0 = 22.797(15) Å, c0 = 9.777(9) Å, β = 115.45(5)°. 相似文献
112.
Crommentuyn KM Rosing H Nan-Offeringa LG Hillebrand MJ Huitema AD Beijnen JH 《Journal of mass spectrometry : JMS》2003,38(2):157-166
HIV protease inhibitors are important antiretroviral drugs which have substantially reduced the morbidity and mortality associated with HIV-1 infection. Recent data have shown relationships between plasma concentrations of the protease inhibitors and clinical response, which makes therapeutic drug monitoring valuable. We have developed and validated an assay, using liquid chromatography coupled with electrospray tandem mass spectrometry (LC/MS/MS), for the routine quantification of the six licensed protease inhibitors (amprenavir, indinavir, lopinavir, nelfinavir, ritonavir and saquinavir) and the pharmacologically active nelfinavir metabolite M8 in plasma. The sample pretreatment consisted of protein precipitation with a mixture of methanol and acetronitrile using only 100 microl of plasma. Chromatographic separation was performed on an Inertsil ODS3 column (50 x 2.0 mm i.d., particle size 5 microm), with a quick stepwise gradient using an acetate buffer (pH 5) and methanol, at a flow rate of 0.5 ml min(-1). The analytical run time was 5.5 min. The use of a 96-well plate autosampler allowed batch sizes up to 150 patient samples. The triple-quadrupole mass spectrometer was operated in the positive ion mode and multiple reaction monitoring was used for drug quantification. The method was validated over the concentration ranges 0.01-10 microg ml(-1) for indinavir and saquinavir, 0.1-10 microg ml(-1) for amprenavir, 0.05-10 microg ml(-1) for nelfinavir and ritonavir, 0.1-20 microg ml(-1) for lopinavir and 0.01-5 microg ml(-1) for M8. Saquinavir-d(5) and indinavir-d(6) were used as internal standards. The coefficients of variation were always <10% for both intra-day and inter-day precisions for each compound. Mean accuracies were also between the designated limits (+/-15%). The validated concentration ranges proved to be adequate in daily practice. This robust and fast LC/MS/MS assay is now successfully applied for routine therapeutic drug monitoring and pharmacokinetic studies in our hospital. 相似文献
113.
U. Berg R. Isaksson J. Sandström U. Sjöstrand A. Eiglsperger A. Mannschreck 《Tetrahedron letters》1982,23(41):4237-4240
(+)- and (?)- as well as (+)- and (?)- were partially separated by liquid chromatography on triacetylcellulose; racemizations resulted in the barriers (Table 2) to rotation via planarized transition states. 相似文献
114.
The crystal structure of a second high-pressure copper vanadate phase, CuVO3(II), has been determined and refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.042 (Rw = 0.051). The space group is rhombohedral, , with hexagonal unit cell a = 4.966(2) and c = 14.084(5) Å [aR = 5.501(2) Å and α = 53.66(3)°]. The structure is the fully ordered ilmenite-type and, on the basis of published magnetic data and the interatomic distances, the valence distribution Cu+V5+O3 is proposed. This represents a unique example of Cu+ in an octahedral environment. 相似文献
115.
Kulova T. L. Karseeva E. I. Skundin A. M. Kachibaya E. I. Imnadze R. A. Paikidze T. V. 《Russian Journal of Electrochemistry》2004,40(5):494-499
A series of compounds with the general formula LiMn2 - x - y
Cr
x
Ni
y
O4, where x + y = 0.05, 0.5, or 1.0, is synthesized. It is shown that all these compounds are pure-phase spinels with parameter aequal to 0.8193-0.8236 nm. Doping a stoichiometric lithium-manganese spinel simultaneously with chromium and nickel makes the spinel structure stable. The initial specific capacity of a spinel depends on its doping degree. Doping LiMn2O4 with chromium and nickel simultaneously at an Mn : Cr : Ni ratio of 195 : 3 : 2 raises the spinel's specific capacity and reduces the cycling degradation. The change in the discharge capacity of LiMn1.95Cr0.03Ni0.02O4 electrodes cycled at 20, 0, and -14°C is determined. 相似文献
116.
Heat transfer from a plasma flow to a metallic or nonmetallic spherical particle is studied in this paper for the extreme case of free-molecule flow regime. Analytical expressions are derived for the heat flux due to, respectively, atoms, ions, and electrons and for the floating potential on the sphere exposed to a two-temperature plasma flow. It has been shown that the local or average heat flux density over the whole sphere is independent of the sphere radius and approximately in direct proportion to the gas pressure. The presence of a macroscopic relative velocity between the plasma and the sphere causes substantially nonuniform distributions of the local heat flux and enhances the total heat flux to the sphere. The heat flux is also enhanced by the gas ionization. Appreciable difference between metallic and nonmetallic spheres is found in the distributions along the oncoming flow direction of the floating potential and of the local heat flux densities due to ions and electrons. The total heat flux to the whole sphere is, however, almost the same for these different spheres. For a fixed value of the electron temperature, the heat flux decreases with increasing temperature ratio Te/Th. 相似文献
117.
Jin Ho Lee Heoung Woo Kim Pyong Ki Pak Hai Bang Lee 《Journal of polymer science. Part A, Polymer chemistry》1994,32(8):1569-1579
Functional group gradient surfaces where the density of functional groups changes gradually along the sample length were prepared. The functional group (? COOH, ? CONH2, and ? OH group) gradient surfaces were produced by the treatment of low-density PE sheets using a corona with gradually increasing power, followed by the graft copolymerization of acrylic acid and subsequent substitution reaction of carboxylic acid groups to amide or hydroxyl groups. The prepared gradient surfaces were characterized by the measurement of water contact angle, FTIR-ATR, and ESCA. The gradient surfaces prepared can be used to systematically investigate the interactions of biological or other species in terms of the surface functional groups and their density of polymeric materials. © 1994 John Wiley & Sons, Inc. 相似文献
118.
Electrical, magnetic, and electron paramagnetic resonance (EPR) measurements have been made on crystals and powders of several quaternary chalcogenides of the type Cu2AIIBIVX4, where AII = Zn, Mn, Fe, or Co, BIV = Si, Ge, or Sn, and X = S or Se. The electrical properties of these compounds are extrinsic, but their magnetic properties do not appear to be affected by impurities. The magnetic moments of the Cu2MnBX4 compounds decrease with increasing covalency of the MnX bond, and those of Cu2FeGeS4 and Cu2CoGeS4 reflect an orbital contribution to the moment. Both the Weiss constants and magnetic ordering temperatures in these compounds show an evolution from antiferromagnetism to ferromagnetism with increasing separation between the moments. Magnetic measurements on single crystals of Cu2MnGeS4, Cu2CoGeS4, and Cu2FeGeS4 indicate that only the latter is anisotropic. EPR measurements on crystals and powders of Cu2ZnGeS4 doped nominally with 0.1% Mn reveal that Mn2+ experiences an axial distortion and that the bond ionicity is the same as in ZnS. 相似文献
119.
Several aspects of past and current studies in the area of iminium salt photochemistry are discussed. Investigations of olefin-iminium salt photoaddition and photocyclization reactions are reviewed and conclusions about electron-transfer pathways for fluorescence quenching and reaction are discussed. The results of recent studies of alcohol and ether photoaddition to 2-phenyl-1-pyrrolinium salts are presented. These C-C bond forming processes occur in moderate yields to produce β-amino alcohol or ether products. In addition, alcohols and ethers serve as efficient quenchers of pyrrolinium salt fluorescence. Rate constants for quenching appear to be dependent upon both the oxidation potential of the alcohols and ethers and the availability of C-H bond α to oxygen. This data along with deuterium isotope effects on quenching combine to suggest a common mechanism for both fluorescence quenching and photoaddition. The nature of this mechanism is tested using the comparative quenching effeciencies of the tertiary alcohols t-butyl alcohol and 1,2,2-trimethyl-1-cyclopropanol. The latter alcohol having a weak C-C bond adjacent to the hydroxyl function quenches the fluorescence of 2-phenyl-1-pyrrolium salts at a rate two orders of magnitude greater than for t-butyl alcohol. The observations made are interpreted in terms of a sequential electron-proton transfer mechanism for quenching and photoaddition. Lastly, the relationship of iminium salt photochemical studies to other investigations of electron-transfer photochemistry is discussed. 相似文献
120.
N.E. Kolobova A.B. Antonova O.M. Khitrova M.Yu. Antipin Yu.T. Struchkov 《Journal of organometallic chemistry》1977,137(1):69-78
The novel complexes CpRe(CCHPh)(CO)2 and Cp2Re2(μ-CCHPh)(CO)4 containing a terminal and a bridging phenylvinylidene ligand respectively and the binuclear complex Cp(CO)2Re[CC(Ph)C(Ph)CH2]Re(CO)2Cp were obtained in the reaction of CpRe(CO)3 with PhCCH.According to an X-ray study of the latter complex the unusual bridging ligand is η1-bonded to one Re atom and η2-bonded to the other. 相似文献