全文获取类型
收费全文 | 47930篇 |
免费 | 7328篇 |
国内免费 | 9755篇 |
专业分类
化学 | 34742篇 |
晶体学 | 976篇 |
力学 | 2950篇 |
综合类 | 589篇 |
数学 | 6174篇 |
物理学 | 19582篇 |
出版年
2024年 | 84篇 |
2023年 | 675篇 |
2022年 | 1210篇 |
2021年 | 1411篇 |
2020年 | 1654篇 |
2019年 | 1579篇 |
2018年 | 1395篇 |
2017年 | 1441篇 |
2016年 | 1793篇 |
2015年 | 1991篇 |
2014年 | 2273篇 |
2013年 | 3338篇 |
2012年 | 3582篇 |
2011年 | 4864篇 |
2010年 | 3490篇 |
2009年 | 3519篇 |
2008年 | 3333篇 |
2007年 | 3022篇 |
2006年 | 2884篇 |
2005年 | 2721篇 |
2004年 | 2963篇 |
2003年 | 2133篇 |
2002年 | 2054篇 |
2001年 | 1697篇 |
2000年 | 1400篇 |
1999年 | 1105篇 |
1998年 | 855篇 |
1997年 | 689篇 |
1996年 | 649篇 |
1995年 | 547篇 |
1994年 | 485篇 |
1993年 | 763篇 |
1992年 | 753篇 |
1991年 | 530篇 |
1990年 | 533篇 |
1989年 | 482篇 |
1988年 | 231篇 |
1987年 | 144篇 |
1986年 | 135篇 |
1985年 | 115篇 |
1984年 | 95篇 |
1983年 | 65篇 |
1982年 | 56篇 |
1981年 | 53篇 |
1980年 | 31篇 |
1979年 | 19篇 |
1978年 | 13篇 |
1976年 | 14篇 |
1975年 | 15篇 |
1971年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
发现某些含二维液晶基元的热致液晶高分子容易产生反向壁织构与含有高强度有错点(│S│大于1)的向列态纹影织构。这类液晶高分子的织构与形态学特点与含一维棒状液晶基元的其他液晶高分子不同。 相似文献
962.
Thermoluminescence glow curves displayed by ethylene-vinyl acetate copolymers after u.v. irradiation at low temperature have been compared to the depolarization thermocurrents (DTC) described in a previous publication, in order to establish a possible correlation between the thermal liberation of the trapped carriers and the intrinsic relaxation phenomena. The detailed analysis of these glow curves has been carried out, either by studying their spectral distributions between 300 and 650 nm, or by the systematic application of the classical method of partial glow and by a method of partial irradiation which consists in irradiating the sample isothermally at different temperatures in several experiments. Two main glow peaks corresponding to distinct spectral emission zones were found. These peaks occur in temperature zones similar to those of appearance of the DTC peaks; the latter were assigned to primary and secondary dipolar relaxations, i.e. α and β type relaxations, respectively. In addition, the β glow peak presents properties comparable with those of the DTC β peak and hence results probably from thermal untrapping, this untrapping being at least partly assisted by short range chain motions in the polymer. The α glow peak, on the other hand, shows little correspondence with the DTC peak and has essentially a superficial origin; it probably arises from two distinct traps associated with impurities or with interface between amorphous and crystalline regions rather than from the polymeric chains themselves. 相似文献
963.
A one-step chromatographic method capable of separating all isomers of polyethylene glycol (PEG)-growth hormone-releasing factor (GRF) (1-29) conjugates was developed. The unmodified GRF (1-29) and seven different isomers of PEG-GRF (1-29) conjugates were separated by using a simple reversed-phase HPLC method depending on the differences of hydrophobicity due to the number and site of PEG attachment. The PEGylation sites of all isomers of PEG-GRF (1-29) conjugates were identified by determining the molecular masses of the Lys-C digested fragments with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. This study is a first report for the separation of all PEG-conjugate isomers and would be useful for further studies to find the promising conjugate by evaluating biological activity and stability of each isomer. 相似文献
964.
The organic nanoparticles of a blue-light-emitting molecule, 1,3-diphenyl-5-(9-anthryl)-2-pyrazuline, were prepared by reprecipitation method using acetonitrile as the solvent for the molecular precursor. Three morphologies, spherical, doughnut-shaped and cubic, could be observed on the silicon substrate forthe nanoparfides by the volume-controlled addition of acetonitrile. The evolution of particle morphology as a function of acetonitrile addition was attributed to the variation of the growth habits of the particles in the different environment. The nanoparticles exhibit the novel photoluminescence spectra as compared to those of monomer and the bulk crystals. 相似文献
965.
966.
Norikazu Nishiyama Yong-Rong Dong Tao Zheng Yasuyuki Egashira Korekazu Ueyama 《Journal of membrane science》2006,280(1-2):603-609
Microporous carbon membranes were prepared on an -alumina support by a pyrolysis of cationic tertiary amine/anionic polymer composites. The precursor solutions contain a thermosetting resorcinol/formaldehyde (RF) polymer and a cationic tertiary amine. Three types of cationic tertiary amines with different chain lengths were used, such as tetramethlammonium bromide (TMAB), tetrapropylammonium bromide (TPAB) and cetyltrimethylammonium bromide (CTAB). A porous structure was produced by a decomposition of the amine and the resulting pores assisted the further gasification of the RF polymer at high temperature. The carbon/alumina membranes have thin and continuous carbon top layers with a thickness of 1 μm. Gas permeation tests were performed using single gases of CO2, O2, N2, CF4, n-C4H10 and i-C4H10, as well as binary mixtures of CH4/n-C4H10 and N2/CF4 at different temperatures between 23 and 150 °C. The carbon membrane prepared using TMAB showed separation factors higher than 650 for the CH4/n-C4H10 mixtures and higher than 8100 for the N2/CF4 mixture. From the permeation of pure gases with different molecular sizes, the pore sizes of the carbon membrane prepared using TMAB, TPAB and CTAB are estimated to be 4.0, 5.0 and larger than 5.5 Å, respectively, indicating that the micropore size of the carbon membranes is controllable by using different amines. 相似文献
967.
968.
Synthesis and Characterization of a Novel Biomaterial: Maleic Anhydride-modified Poly(dl-lactic acid) 总被引:2,自引:0,他引:2
YanFengLUO YuanLiangWANG XuFengNIU JunPAN LiangPingSHI 《中国化学快报》2004,15(5):521-524
A novel modified poly(dl-lactic acid) (PDLLA) was obtained by covalently grafting of maleic anhydride onto the backbone of PDLLA, attempting to improve PDLLA‘s hydrophilicity and cell affinity and to provide reactive groups for further chemical modification. FTIR, ^13 C NMR and DSC were used to characterize the maleic anhydride-modified PDLLA. 相似文献
969.
在碱性条件下, 聚乙二醇单甲醚(HO\|OPEG\|OMe)与1,4-二氟蒽醌进行亲核单取代反应生成中间体F\|AQN-OPEG-OMe, 产率88%. F-AQN-OPEG-OMe与奎宁锂进行亲核取代反应, 以95.6%的产率得到新型手性配体QN\|AQN\|OPEG-Me. QN-QN\|OPEG-Me与OsO4原位配位生成的均相催化剂在4种烯烃的不对称双羟化反应中表现出较高的对映选择性(92%~99%e.e.)和化学产率(80%~94%). 催化活性和立体选择性与Sharpless手性配体(DHQ)2AQN相当. 反应结束后, 配体可用乙醚沉淀回收, 循环使用5次, 催化剂的催化活性和立体选择性无明显改变, 配体的回收率均在95%~97%之间. 相似文献
970.
Huang KW Han JH Cole AP Musgrave CB Waymouth RM 《Journal of the American Chemical Society》2005,127(11):3807-3816
Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium. 相似文献