全文获取类型
收费全文 | 11220篇 |
免费 | 577篇 |
国内免费 | 145篇 |
专业分类
化学 | 7805篇 |
晶体学 | 101篇 |
力学 | 573篇 |
综合类 | 3篇 |
数学 | 1417篇 |
物理学 | 2043篇 |
出版年
2024年 | 15篇 |
2023年 | 98篇 |
2022年 | 193篇 |
2021年 | 526篇 |
2020年 | 417篇 |
2019年 | 506篇 |
2018年 | 518篇 |
2017年 | 417篇 |
2016年 | 695篇 |
2015年 | 490篇 |
2014年 | 659篇 |
2013年 | 1172篇 |
2012年 | 916篇 |
2011年 | 843篇 |
2010年 | 600篇 |
2009年 | 489篇 |
2008年 | 541篇 |
2007年 | 469篇 |
2006年 | 339篇 |
2005年 | 344篇 |
2004年 | 215篇 |
2003年 | 209篇 |
2002年 | 172篇 |
2001年 | 92篇 |
2000年 | 60篇 |
1999年 | 69篇 |
1998年 | 45篇 |
1997年 | 57篇 |
1996年 | 42篇 |
1995年 | 40篇 |
1994年 | 44篇 |
1993年 | 25篇 |
1992年 | 41篇 |
1991年 | 37篇 |
1990年 | 36篇 |
1989年 | 37篇 |
1988年 | 37篇 |
1987年 | 35篇 |
1986年 | 30篇 |
1985年 | 44篇 |
1984年 | 37篇 |
1983年 | 21篇 |
1982年 | 28篇 |
1981年 | 25篇 |
1980年 | 29篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 15篇 |
1976年 | 18篇 |
1974年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
81.
Well‐defined graft copolymers with styrene butadiene rubber (SBR) backbones and polystyrene branches were synthesized by living free radical polymerization (LFRP) techniques. Thus 1‐ benzoyl‐2‐phenyl‐2‐(2′,2′,6′,6′‐tetramethyl‐piperidinyl‐1′‐oxy)ethane (BZ‐TEMPO) was synthesized and hydrolyzed to the corresponding 1‐hydroxyl derivative. This functional nitroxyl compound was coupled with brominated SBR (SBR‐Br). The resulting macroinitiator (SBR‐TEMPO) for “living” free radical polymerization was then heated in the presence of styrene for the formation of the controlled graft copolymer. 1H‐NMR and IR spectroscopy were used to investigate the structure of the polymers. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
82.
Spectrophotometric Determination of Thorium with Disodium Salt of Arsenazo-III in Perchloric Acid 总被引:1,自引:0,他引:1
M. Haleem Khan A. Ali N. Niaz Khan 《Journal of Radioanalytical and Nuclear Chemistry》2001,250(2):353-357
A rapid and sensitive spectrophotometric method has been developed for the determination of thorium using 0.04% Arsenazo-III in a 2M perchloric acid solution. Absorbance was measured in 1 cm cell and the complex has a sensitive absorption peak at 654 nm. The complex is formed instantly in perchloric acid and remains stable for 45 minutes with constant absorbance. Beer's law is obeyed in the range 1–60 g·g–1 of thorium concentration with a molar absorptivity at 654 nm = 3.07·105 M–1·cm–1 at 24±2°C. The foreign ions interference in thorium determination have been checked. The cations were tested at >60-fold excess of thorium, Mn(II), Fe(III), Co(II) and Ni(II) interfere negatively, whereas only Ce(III) has increased the absorbance. Among the anions, cyanide, phosphate, thiocyanate and acetate at 150-fold excess of thorium cause significant interference. However, thorium can bedetermined in the presence of nitrate, chloride, oxalate, tartrate, ascorbate, thiosulphate and citrate. The method has been applied on certified reference material for thorium determination after extractive separation and the result was found in good agreement with the certified value. The method has been also applied successfully to determine thorium at g·g–1 level in local ore samples with a precision of ±0.04%. 相似文献
83.
Garraffo HM Spande TF Jain P Kaneko T Jones TH Blum MS Ali TM Snelling RR Isbell LA Robertson HG Daly JW 《Rapid communications in mass spectrometry : RCM》2001,15(16):1409-1415
Chemical ionization tandem mass spectrometry (CI-MS/MS) of alkaloids with ammonia reagent gas and collision-activated dissociation as well as EI-MS/MS were applied to the tetraponerine alkaloids in extracts from six pseudomyrmecine ants of the genus Tetraponera. The MS/MS techniques along with gas chromatography Fourier transform infrared (GC/FTIR) spectra allowed identification in two extracts of seven of the eight known tetraponerines. The EI-MS/MS fragmentations proved diagnostic for the ring system and the CI-MS/MS patterns for the C-8 or C-9 substitution, while the Bohlmann bands in FTIR spectra were diagnostic for the C-8 or C-9 configurations. An Indian ant (T. allaborans) had T-2, T-4 and T-8, while a Chinese ant (T. binghami) had T-5, T-6, T-7 and T-8. Four other ants, T. rufonigra (India), T. penzigi (Africa), T. clypeata (Africa) and T. sp. cf. emeryi (Africa), had no tetraponerines. 相似文献
84.
Ali Liazid Gerardo F. Barbero Miguel Palma Jamal Brigui Carmelo G. Barroso 《Chromatographia》2007,66(7-8):571-575
Results from a systematic study of the factors affecting extraction of cis-verbenol and verbenone from pine seeds are presented. Five extraction conditions were investigated: extraction solvent, method
of extraction, extraction temperature, volume of solvent, and the ratio of the mass of sample to the amount of extraction
solvent. The resulting optimized method uses magnetic-stirring-assisted extraction of pine seeds (5 g) with ethyl acetate
(75 mL) for 20 min, at room temperature. RSDs were less than 5% for both compounds. GC–FID was used for quantification of
cis-verbenol and verbenone in the extracts. 相似文献
85.
A mixed-ligand Zn(II) complex formulated as [Zn(aldtc)2(bipy)] (aldtc=diallyldithiocarbamate; bipy=2,2′-bipyridine) was synthesized and characterized by IR, 1H and 13C NMR spectral measurements and X-ray crystallography. The crystal structure of this complex indicates that Zn has a distorted octahedral geometry. The Zn—N distances are invariant (2.168(2) Å), while those of the Zn—S are slightly different (2.5408(9) and 2.5440(9) Å). The N—Zn—N, S—Zn—S and N—Zn—S bond angles are in the range 75.35(13)–99.75(7)°, 70.48(3)–161.02(5)° and 95.26(7)–160.32(7)°, respectively. The crystal packing of the complex shows different motifs of supramolecularity resulting from both hydrophilic ((π)C—H···S) and hydrophobic ((allyl)C—H···C(π)) intermolecular interactions. These interactions result in a chain arrangement of molecules along crystallographic c axis and the chains are further connected via π···π stacking along with ((π)C—H···S along b axis leading to an overall crystal packing that can be regarded as layers of complexes along bc plane, which are held together through nonconventional hydrogen bonding and π···π stacking. 相似文献
86.
Sonia Jam Masume Ghalbi Ahangary Ahmad Tavasoli Kambiz Sadaghiani Ali Nakhaei Pour 《Reaction Kinetics and Catalysis Letters》2006,89(1):71-79
Summary An integrated process for producing liquid fuels from synthesis gas via a two-stage Fischer-Tropsch (FT) reaction is disclosed. An iron catalyst was used in the first bed of a fixed-bed reactor
followed by a ruthenium promoted cobalt catalyst in the second bed. The activity and selectivity of the dual-bed system were
assessed and compared with those using catalysts in a single bed system, separately. The methane selectivity in the dual-bed
reactor was about 11% less compared to that of the single-bed system. The C5+ selectivity for the dual-bed reactor was 19.7% higher than that of the single-bed system. 相似文献
87.
The reactions of ten main group acidic oxides and oxyanions with molten alkali metal carbonate eutectics have been studied by thermogravimetry. Products have been identified by IR and Raman spectroscopy and stoichiometries suggested for the several reactions as oxide is progressively incorporated. Ultimately the tetrahedral ortho-oxyanion was formed except with the less acidic oxides (As2O3, Sb2O3, SiO2). Sulphite was partially oxidised by the carbon dioxide atmosphere to sulphate. 相似文献
88.
Sayed Ali Naghi Taheri R. Alan Jones Santokh Singh Badesha Majed M. Hania 《Tetrahedron》1989,45(24):7717-7728
3,4-Diformyl-2,5-dimethylpyrrole (1) reacts with ,ω-diamino-alkanes, NH2(CH2)nNH2t' to form either the potentially tautomeric 2:2 macrocyclic adduct (7a) (8), when N = 2, or the potentially tautomeric 1:1 bicyclic adduct (18) (19), when N = 4, 5, 6, and 12. 1H and 13C N.m.r. spectral data indicate that the 2-azafulvene structures predominate for both types of cycloadducts. Only polymeric material was obtained when N = 3. 相似文献
89.
Relative rates of solvolysis of some N-triorganosilylanilines in mixtures of ethanol and aqueous potassium hydroxide have been determined, with results as follows. (i) For XC6H4NHSiEt3 compounds in MeOH (5 vol) + aq. alkali (2 vol) at 50°: (X =) H, 1.0;p-Me, 0.80;p-OMe, 0.83;m-Me, 0.90; o-Me, 0.87; p-SMe, 1.90; p-F, 1.7; p-Cl, 2.8; o-Cl, 14; m-Cl, 4.2; m-NO2, 18; p-CN, ca. 43; p-NO2, ca. 120. (ii) For PhNHSi(C6H4Y)3 compounds in MeOH (10 vol) + aq. alkali (1 vol) at 50°: (Y =) H, 1.0; p-OMe, 0.12; p-Cl, ca. 32; m-Cl, ca. 84. (iii) For PhNHSiR3 compounds in MeOH (5 vol) + aq. alkali (2 vol) : (R3 =) Et3, 1.0; Et2Me, 18 (at 30°); Me2-i-Pr, 8 (at 30°);Me2-t-Bu, 0.012 (at 50°);i-Pr3, 0.006 (at 50°). In series (i) the relative rates correlate with σ, or where appropriate σ?-constants, with a ? value of 1.6. It is suggested that in the transition state of the rate-determining step the OSi bond is fully formed, or almost so, the SiN bond approximately 20—50% broken, and the bond between the nitrogen atom and a proton from the solvent ca. 10—30% formed. 相似文献
90.
Mohammad Ali Zolfigol Iraj Mohammadpoor-BaltorkDavood Habibi BiBi Fatemeh MirjaliliAbdolhamid Bamoniri 《Tetrahedron letters》2003,44(44):8165-8167
Primary and secondary trimethylsilyl ethers were converted to their corresponding ethers in the presence Nafion-H® with good to excellent yields under mild and heterogeneous conditions. 相似文献