全文获取类型
收费全文 | 40803篇 |
免费 | 6668篇 |
国内免费 | 5586篇 |
专业分类
化学 | 31054篇 |
晶体学 | 607篇 |
力学 | 2076篇 |
综合类 | 365篇 |
数学 | 4455篇 |
物理学 | 14500篇 |
出版年
2024年 | 97篇 |
2023年 | 746篇 |
2022年 | 1201篇 |
2021年 | 1419篇 |
2020年 | 1623篇 |
2019年 | 1682篇 |
2018年 | 1435篇 |
2017年 | 1423篇 |
2016年 | 1928篇 |
2015年 | 1973篇 |
2014年 | 2493篇 |
2013年 | 3138篇 |
2012年 | 3710篇 |
2011年 | 3937篇 |
2010年 | 2929篇 |
2009年 | 2694篇 |
2008年 | 2930篇 |
2007年 | 2675篇 |
2006年 | 2488篇 |
2005年 | 2039篇 |
2004年 | 1508篇 |
2003年 | 1224篇 |
2002年 | 1151篇 |
2001年 | 942篇 |
2000年 | 864篇 |
1999年 | 719篇 |
1998年 | 563篇 |
1997年 | 499篇 |
1996年 | 445篇 |
1995年 | 392篇 |
1994年 | 415篇 |
1993年 | 292篇 |
1992年 | 267篇 |
1991年 | 229篇 |
1990年 | 206篇 |
1989年 | 173篇 |
1988年 | 117篇 |
1987年 | 83篇 |
1986年 | 94篇 |
1985年 | 76篇 |
1984年 | 40篇 |
1983年 | 29篇 |
1982年 | 33篇 |
1981年 | 13篇 |
1980年 | 23篇 |
1979年 | 13篇 |
1976年 | 11篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
991.
新型水溶性膦铑络合物催化烯烃的氢甲酰化反应研究 总被引:10,自引:1,他引:10
新型水溶性膦铑络合物催化烯烃的氢甲酰化反应研究燕远勇,左焕培,金子林(大连理工大学化工学院,大连116012)关键词烯烃,水溶性膦铑络合催化剂,氢甲酰化,醛.1.前言为克服催化剂的流失和与反应产物的分离困难,近年来均相催化的一个重要进展是开发了以磺化... 相似文献
992.
993.
Two cobalt(II) coordination polymers formed from bte (bte = 1,2-bis(1,2,4-triazol-1-yl)ethane), namely [Co(bte)2(dca)2]n (1) and {[Co(bte)(dca)2] · H2O}n (2), have been synthesized and characterized by elementary analyses, IR, thermogravimetric analyses, X-ray diffraction analyses and magnetic measurements. Compound 1 is a double-chain with Co(II) centers bridged by bte, containing metallocycles of [Co2(bte)2] and trans dca as termination ligands. In 2, each Co(II) center is bonded by two bridging bte ligands and four dca as μ-1,5-dca in different orientations in the 3D network. 相似文献
994.
Melt extrusion was used to prepare binary nanocomposites of ethylene copolymers and organoclay and trinary nanocomposites of low-density polyethylene (LDPE), ethylene copolymer and organoclay. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to analyze the structure of the clay phase and the morphology of the nanocomposites. Influences of the comonomer in the copolymer and the content of the copolymer on the morphology of the resulting nanocomposites were discussed. The binary and the trinary composites may form intercalated or exfoliated structures depending on the interaction between the copolymer and the clay layers and the content of the copolymer. 相似文献
995.
Cerium-doped mesoporous TiO2 nanoparticles with high surface area and thermal stable anatase wall were synthesized via hydrothermal process in a cetyltrimethylammonium bromide (CTAB)/Ti(SO4)2/Ce(NO3)4/H2O system. The obtained materials were characterized by XRD, FESEM, HRTEM, FTIR spectroscopy, nitrogen adsorption and DRS spectra. Experimental results indicated that the doping of cerium not only increased the surface area of mesoporous TiO2 nanoparticles, but also inhibited the mesopores collapse and the anatase-to-rutile phase transformation. Moreover, the undoped, doped anatase mesoporous nanoparticles exhibit higher photocatalytic activity than commercial photocatalyst (Degussa, P25), but the maximum photodegradation rate corresponds to the undoped mesoporous TiO2 nanoparticles. The lower photocatalytic activities of cerium-doped samples compared with undoped one may be ascribed to that the doped cerium partially blocks titania's surface sites available for the photodegradation and absorption of Rhodamine B (RB). 相似文献
996.
Instead of environmentally toxic chromium oxidant, singlet oxygen generated photcchemically was used as environmentally friendly and benign oxidizing agent to accomplish the transformation of pseudodiosgenin diacetate to diosone efficiently in a low toxic and less expensive solvent acetone. Accordingly 16-dehydropregnenolone acetate(16-DPA), an important intermediate for preparation of steroidal drugs, was prepared in good yield (75%) when photoreaction was run in acetone/acetic anhydride/pyridine system. The mild reaction condition as well as simple and environmentally friendly process made the method commercially viable and important for production of 16-DPA in industrial scale. 相似文献
997.
The interactions between drug molecules and membrane were studied using the new chromatography stationary phase of liposome coated zirconia–magnesia. log Ks(ZrO2–MgO) on this new chromatography for some drugs, compared with that on liposome coated silica chromatography and other reported data, fair correlations were observed between them when excluding effect of special adsorption. log Ks(ZrO2–MgO) values for barbitalum, diazepam, benzene, benzocaine and toluene correlated well with corresponding values on liposome coated silica chromatography (R = 0.99778, P < 0.001; R = 0.98229, P < 0.003; R = 0.9985, P < 0.0001; R = 0.99925, P < 0.0001, pH value of mobile phase at pH 7.4, 7.0, 6.4 and 5.4, respectively). They also correlated well with the literature data on immobilized artificial membrane chromatography (R = 0.99999, P < 0.004 at pH 7.4) and liposome chromatography (R = 0.99994, P < 0.008) for procaine, lidocaine and bupivacaine. Liposome coated zirconia–magnesia chromatography can thus be used for studying drug–membrane interaction and prediction of drug absorption as another liposome chromatography method. 相似文献
998.
Song W Justice RE Jones CA Grassian VH Larsen SC 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):8301-8306
Nanocrystalline ZSM-5 with a Si/Al ratio of 20 was synthesized using clear solutions and a hydrothermal synthesis procedure. The resulting ZSM-5 materials were characterized by powder X-ray diffraction, scanning electron microscopy (SEM), nitrogen adsorption isotherms, solid-state nuclear magnetic resonance, and toluene adsorption. A commercial ZSM-5 sample was similarly characterized for comparison with the synthesized materials. The particle sizes of the synthesized ZSM-5 samples were calculated using the measured external surface areas and were determined to be 15 and 60 nm. SEM images indicated that the ZSM-5 samples consist of agglomerated and possibly intergrown particles. Toluene adsorption measurements showed that the ZSM-5 sample with a particle size of 15 nm adsorbed approximately 50% more toluene than the other ZSM-5 samples, most likely due to the adsorption of toluene on the external surface. For the toluene adsorbed on the internal zeolite surface, approximately one toluene molecule was adsorbed per channel intersection for each of the ZSM-5 samples. 相似文献
999.
考察了阳离子、非离子和阴离子表面活性剂存在下水杨酸-2’-乙基己基酯(EHS)的吸收光谱和激发态分子内质子转移(ESIPT)荧光光谱. 结果表明, EHS可增溶在胶束中, 2’-乙基己基碳链朝向胶束内核, 而水杨酸基朝向胶束-水界面; 胶束环境有利于EHS分子对紫外光的吸收和分子内氢键的形成, 从而使ESIPT 荧光显著增强, 激发态分子以发射可见光和非辐射去活化方式衰减; 并根据EHS和表面活性剂分子的结构和大小, 解释了EHS分子在胶束中的结合位点, 荧光猝灭和酯水解的光谱测量进一步为此结合位点提供了佐证. 相似文献
1000.
Ping Wang Yi-Zeng Liang Ben-Mei Chen Neng Zhou Lun-Zhao Yi Yan Yu Zhi-Biao Yi 《Chromatographia》2007,65(3-4):209-215
A rapid, simple and specific liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method has been developed
and validated for the simultaneous determination of enalapril and its major active metabolite enalaprilat in human plasma.
Benazepril hydrochloride was used as the internal standard. Plasma was deproteinized with acetone and centrifuged. The supernatant
was transferred and evaporated to dryness and the residue dissolved in mobile phase. Samples were separated on a C18 column
with a mobile phase of methanol–20 mM ammonium acetate (53:47, v/v) containing 0.15% trifluoracetic acid (v/v) with a pH of 3.0. Enalapril, enalaprilat and the internal standard were measured by electrospray positive selective ion
monitoring mode. The method was validated over a linear range of 1.56–400 ng mL−1 and the limits of quantification were 1.56 ng mL−1 for both enalapril and enalaprilat using 0.1 mL plasma. Extraction efficiency was more than 84% and recoveries were in range
of 93.65–101.17%. The intra-day relative standard deviations (RSD) were less than 8.16 and 7.05% and inter-day RSDs were within
8.42 and 5.72% for enalapril and enalaprilat, respectively. The storage stability of QC samples was investigated under various
conditions. The method was successfully applied for the evaluation of the pharmacokinetics and bioequivalence of enalapril
and enalaprilat in 20 healthy volunteers after an oral dose of 20 mg enalapril maleate. 相似文献