全文获取类型
收费全文 | 765篇 |
免费 | 36篇 |
国内免费 | 4篇 |
专业分类
化学 | 627篇 |
晶体学 | 7篇 |
力学 | 9篇 |
数学 | 96篇 |
物理学 | 66篇 |
出版年
2023年 | 4篇 |
2022年 | 17篇 |
2021年 | 21篇 |
2020年 | 26篇 |
2019年 | 17篇 |
2018年 | 13篇 |
2017年 | 10篇 |
2016年 | 32篇 |
2015年 | 37篇 |
2014年 | 20篇 |
2013年 | 40篇 |
2012年 | 50篇 |
2011年 | 61篇 |
2010年 | 39篇 |
2009年 | 28篇 |
2008年 | 39篇 |
2007年 | 51篇 |
2006年 | 41篇 |
2005年 | 37篇 |
2004年 | 32篇 |
2003年 | 29篇 |
2002年 | 18篇 |
2001年 | 9篇 |
2000年 | 13篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1923年 | 1篇 |
排序方式: 共有805条查询结果,搜索用时 11 毫秒
121.
Hausdorf S Wagler J Mossig R Mertens FO 《The journal of physical chemistry. A》2008,112(33):7567-7576
The contributions of terephthalic acid and Zn(2+)-coordinated water in N,N-diethylformamide (DEF) to the overall proton activity in the synthesis of MOF-5 (Zn4O(BDC)3, BDC = 1,4-benzenedicarboxylate) were quantitatively determined by combined electrochemical and UV-vis spectroscopic measurements. Structural transformations of zinc carboxylate-based metal organic frameworks due to their exposure to environments with variable water concentrations and the chemical means necessary to revert these transitions have been investigated. It was found that the water-induced structural transition of MOF-5 to the hydroxide structure Zn3(OH)2(BDC)2 x 2 DEF (MOF-69c) can be reverted by a thermal treatment of the obtained compound and its subsequent exposure to anhydrous DEF. MOF-5 syntheses from simple carboxylates as well as a water-free synthesis based on nitrate decomposition are presented. The latter demonstrates that nitrate can serve as the sole source for the oxide ion in MOF-5. 相似文献
122.
Regina Pamplona Sandra González-Lana Pilar Romero Ignacio Ochoa Rafael Martín-Rapún Carlos Sánchez-Somolinos 《Macromolecular bioscience》2023,23(12):2300227
From the first experiments with biomaterials to mimic tissue properties, the mechanical and biochemical characterization has evolved extensively. Several properties can be described, however, what should be essential is to conduct a proper and physiologically relevant characterization. Herein, the influence of the reaction media (RM) and swelling media (SM)–phosphate buffered saline (PBS) and Dulbecco's modified Eagle's medium (DMEM) with two different glucose concentrations–is described in gelatin methacrylamide (GelMA) hydrogel mechanics and in the biological behavior of two tumoral cell lines (Caco-2 and HCT-116). All scaffolds are UV-photocrosslinked under identical conditions and evaluated for mass swelling ratio and stiffness. The results indicate that stiffness is highly susceptible to the RM, but not to the SM. Additionally, PBS-prepared hydrogels exhibited a higher photopolymerization degree according to high resolution magic-angle spinning (HR-MAS) NMR. These findings correlate with the biological response of Caco-2 and HCT-116 cells seeded on the substrates, which demonstrated flatter morphologies on stiffer hydrogels. Overall, cell viability and proliferation are excellent for both cell lines, and Caco-2 cells displayed a characteristic apical-basal polarization based on F-actin/Nuclei fluorescence images. These characterization experiments highlight the importance of conducting mechanical testing of biomaterials in the same medium as cell culture. 相似文献
123.
Regina J. Patinvoh Elisabeth Feuk-Lagerstedt Magnus Lundin Ilona Sárvári Horváth Mohammad J. Taherzadeh 《Applied biochemistry and biotechnology》2016,180(7):1401-1415
Chicken feathers are available in large quantities around the world causing environmental challenges. The feathers are composed of keratin that is a recalcitrant protein and is hard to degrade. In this work, chicken feathers were aerobically pretreated for 2–8 days at total solid concentrations of 5, 10, and 20 % by Bacillus sp. C4, a bacterium that produces both α- and β-keratinases. Then, the liquid fraction (feather hydrolysate) as well as the total broth (liquid and solid fraction of pretreated feathers) was used as substrates for biogas production using anaerobic sludge or bacteria granules as inoculum. The biological pretreatment of feather waste was productive; about 75 % of feather was converted to soluble crude protein after 8 days of degradation at initial feather concentration of 5 %. Bacteria granules performed better during anaerobic digestion of untreated feathers, resulting in approximately two times more methane yield (i.e., 199 mlCH4/gVS compared to 105 mlCH4/gVS when sludge was used). Pretreatment improved methane yield by 292 and 105 % when sludge and granules were used on the hydrolysate. Bacteria granules worked effectively on the total broth, yielded 445 mlCH4/gVS methane, which is 124 % more than that obtained with the same type of inoculum from untreated feather. 相似文献
124.
Merlin Rosales Karely Molina Regina Vallejo Edgar Ocando-Mavárez 《Transition Metal Chemistry》2016,41(4):467-473
The complex Rh(acac)(CO)[P(tBu)(CH2CH=CH2)2] (1) proved to be an efficient precatalyst for the regioselective hydrogenation of quinoline (Q) to 1,2,3,4-tetrahydroquinoline (THQ) under mild reaction conditions (125 °C and 4 atm H2). A kinetic study of this reaction led to the rate law: which becomes at hydrogen pressures below 4 atm. The active catalytic species is the cationic complex {Rh(Q)2(CO)[P(tBu)(CH2CH=CH2)2]}+ (2). The mechanism involves the partial hydrogenation of one coordinated Q of (2) to yield a complex containing a 1,2-dihydroquinoline (DHQ) ligand, {Rh(DHQ)(Q)(CO)[P(tBu)(CH2CH=CH2)2]}+ (3), followed by hydrogenation of the DHQ ligand to give THQ and a coordinatively unsaturated species {Rh(Q)(CO)[P(tBu)(CH2CH=CH2)2]}+ (4); this reaction is considered to be the rate-determining step. Coordination of a new Q molecule to (4) regenerates the active species (2) and restarts the catalytic cycle.
相似文献
$$ r \, = \{ K_{1} k_{2} /(1 \, + \, K_{1} {\text{H}}_{ 2} )\} [{\text{Rh}}][{\text{H}}_{ 2} ]^{2} $$
$$ r \, = \, K_{1} k_{2} [{\text{Rh}}][{\text{H}}_{ 2} ]^{2} $$
125.
Teixeira RR Barbosa LC Maltha CR Rocha ME Bezerra DP Costa-Lotuf LV Pessoa C Moraes MO 《Molecules (Basel, Switzerland)》2007,12(5):1101-1116
3-Benzyl-furan-2(5H)-one (2a) and 3-(4-bromobenzyl)-furan-2(5H)-one (2b) were treated with TBDMSOTf and converted into the corresponding tert-butyldimethyl-silylfuran ethers. These furans were further condensed with several aromatic aldehydes affording compounds 5-14 with general 3-benzyl-5-arylidene-furan-2(5H)-one structures in 31% to 98% yields. Such compounds are analogues of the naturally occurring nostoclide lactones, reported to present moderate cytotoxic activity. Compounds 5-14 were submitted to an in vitro bioassay against the HL-60, HCT-8, SF295 and MDA-MB-435 cancer cell lines using the MTT cytotoxicity assay. 相似文献
126.
Gerard B Cencic R Pelletier J Porco JA 《Angewandte Chemie (International ed. in English)》2007,46(41):7831-7834
127.
Cavalcanti ED Maciel FM Villeneuve P Lago RC Machado OL Freire DM 《Applied biochemistry and biotechnology》2007,137(1-12):57-65
The influence of several factors on the hydrolytic activity of lipase, present in the acetone powder from dormant castor seeds (Ricinus communis) was evaluated. The enzyme showed a marked specificity for short-chain substrates. The best reaction conditions were an acid medium, Triton X-100 as the emulsifying agent and a temperature of 30 degrees C. The lipase activity of the acetone powder of different castor oil genotypes showed great variability and storage stability of up to 90%. The toxicology analysis of the acetone powder from genotype Nordestina BRS 149 showed a higher ricin (toxic component) content, a lower 2S albumin (allergenic compound) content, and similar allergenic potential compared with untreated seeds. 相似文献
128.
Respondek M Madl T Göbl C Golser R Zangger K 《Journal of the American Chemical Society》2007,129(16):5228-5234
Many antimicrobial peptides form alpha-helices when bound to a membrane. In addition, around 80% of residues in membrane-bound proteins are found in alpha-helical regions. The orientation and location of such helical peptides and proteins in the membrane are key factors determining their function and activity. Here we present a new solution state NMR method for obtaining the orientation of helical peptides in a membrane-mimetic environment (micelle-bound) without any chemical perturbation of the peptide-micelle system. By monitoring proton longitudinal relaxation rates upon addition of the freely water-soluble and inert paramagnetic probe Gd(DTPA-BMA) to an alpha-helical peptide, a wavelike pattern with a periodicity of 3.6 residues per turn is observed. The tilt and azimuth (rotation) angle of the helix determine the shape of this paramagnetic relaxation wave and can be obtained by least-square fitting of measured relaxation enhancements. Results are presented for the 15-residue antimicrobial peptide CM15 which forms an amphipathic helix almost parallel to the surface of the micelle. Thus, a few fast experiments enable the identification of helical regions and determination of the helix orientation within the micelle without the need for covalent modification, isotopic labeling, or sophisticated equipment. This approach opens a path toward the topology determination of alpha-helical membrane-proteins without the need for a complete NOE-based structure determination. 相似文献
129.
In S Orlov A Berg R García F Pedrosa-Jimenez S Tikhov MS Wright DS Lambert RM 《Journal of the American Chemical Society》2007,129(45):13790-13791
130.
A simple, sensitive, and specific biodiffusion assay for the antibacterial ceftazidime was developed using a strain of Staphylococcus epidermidis (ATCC 12228) as the test organism. Ceftazidime was measured in powder for injection at concentrations ranging from 100 to 400 microg/mL. The calibration graph for ceftazidime was linear (r2 = 1), and the method validation showed that it was precise (relative standard deviation = 0.415) and accurate. The results obtained by biodiffusion assay were statistically calculated by linear parallel model and by means of regression analysis and were verified using analysis of variance. It was concluded that the microbiological assay is satisfactory for in vitro quantification of the antibacterial activity of ceftazidime in pharmaceuticals. 相似文献