全文获取类型
收费全文 | 17239篇 |
免费 | 2631篇 |
国内免费 | 1868篇 |
专业分类
化学 | 9194篇 |
晶体学 | 124篇 |
力学 | 899篇 |
综合类 | 92篇 |
数学 | 1636篇 |
物理学 | 5117篇 |
无线电 | 4676篇 |
出版年
2024年 | 74篇 |
2023年 | 453篇 |
2022年 | 530篇 |
2021年 | 698篇 |
2020年 | 659篇 |
2019年 | 567篇 |
2018年 | 552篇 |
2017年 | 501篇 |
2016年 | 699篇 |
2015年 | 788篇 |
2014年 | 929篇 |
2013年 | 1228篇 |
2012年 | 1370篇 |
2011年 | 1378篇 |
2010年 | 1012篇 |
2009年 | 954篇 |
2008年 | 1063篇 |
2007年 | 1003篇 |
2006年 | 939篇 |
2005年 | 760篇 |
2004年 | 530篇 |
2003年 | 459篇 |
2002年 | 436篇 |
2001年 | 393篇 |
2000年 | 349篇 |
1999年 | 416篇 |
1998年 | 362篇 |
1997年 | 293篇 |
1996年 | 342篇 |
1995年 | 300篇 |
1994年 | 264篇 |
1993年 | 213篇 |
1992年 | 185篇 |
1991年 | 190篇 |
1990年 | 166篇 |
1989年 | 123篇 |
1988年 | 109篇 |
1987年 | 83篇 |
1986年 | 66篇 |
1985年 | 65篇 |
1984年 | 33篇 |
1983年 | 36篇 |
1982年 | 35篇 |
1981年 | 25篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1974年 | 9篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
在硼氢化钾碱性溶液中对金属氢化物(MH)电极的表面进行化学还原处理,提高了MH电极的放电容量、活化性能和电催化活性.用其为负极组装的AA型MH-Ni电池进行了封口化成,电池放电容量达到1150mAh,5C下电池的放电容量达到0.2C下容量的80%以上,电池在1C100%DOD(放电深度)充放电条件下,循环寿命由原来的100次左右提高到200次以上 相似文献
172.
Zhi C Bando Y Tang C Xie R Sekiguchi T Golberg D 《Journal of the American Chemical Society》2005,127(46):15996-15997
We report for the first time that boron nitride nanotubes (BNNTs) may be dissolved in organic solvents by wrapping them with a polymer. Transmission electron microscopy and cathodoluminescence studies indicate the strong pi-pi interactions between BNNTs and the polymer. A band gap ranging from 5.2 to 5.5 eV was documented for the BNNTs independent of their geometrical characteristics by using ultraviolet-visible absorption experiments on composite films and thin BNNT films prepared from solutions. 相似文献
173.
Two molecular imprinting polymer (MIP) monolithic columns with (S)-(-)-1,1'-bi-2-naphthol and (R)-(+)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol as the templating molecules, respectively, have been prepared by in situ polymerization using 4-vinylpyridine and ethylene dimethacrylate as functional monomer and cross-linker, respectively. The columns with good flow-through properties were obtained by changing the molar ratio of the functional monomer and the template molecule. The effects of mobile-phase composition on separation of enantiomers were systematically investigated. The results indicate that hydrophobic interaction in aqueous solution and hydrogen-bonding interaction in ACN between the enantiomers and polymers could play important roles in the retention and resolution. The effects of chromatographic conditions, such as flow rate, column temperature, sample loading, on the enantioseparation were also studied. Further, these two MIP columns show a cross-reactivity. 相似文献
174.
A series of 2-benzyl-1,3-dicabonyl derivatives was synthesized.Their insulin-sensitizing activity was evaluated in 3T3-L1 preadipocyte cells.Compounds3,26 and 27 were found to possess strong insulin-sensitizing activity in vitro and were selected for further hypoglycemic evaluation in vivo. 相似文献
175.
Samuel K Yin W Stearns RA Tang YS Chaudhary AG Jewell JP Lanza T Lin LS Hagmann WK Evans DC Kumar S 《Journal of mass spectrometry : JMS》2003,38(2):211-221
Metabolic activation of drug candidates to electrophilic reactive metabolites that can covalently modify cellular macromolecules may result in acute and/or idiosyncratic immune system-mediated toxicities in humans. This presents a significant potential liability for the future development of these compounds as safe therapeutic agents. We present here an example of an approach where sites of metabolic activation within a new drug candidate series were rapidly identified using online liquid chromatography/multi-stage mass spectrometry on an ion trap mass spectrometer. This was accomplished by trapping the reactive intermediates formed upon incubation of compounds with rat and human liver microsomes as their corresponding glutathione conjugates and mass spectral characterization of these thiol adducts. Based on the structures of the GSH adducts identified, potential sites and mechanisms of bioactivation within the chemical structure were proposed. These metabolism studies were interfaced with iterative structural modifications of the chemical series in order to block these bioactivation sites within the molecule. This strategy led to a significant reduction in the propensity of the compounds to undergo metabolic activation as evidenced by reductions in the irreversible binding of radioactivity to liver microsomal material upon incubation of tritium-labeled compounds with this in vitro system. With the efficiency and throughput achievable with such an approach, it appears feasible to identify and address the metabolic activation potential of new drug leads during routine metabolite identification studies in an early drug discovery setting. 相似文献
176.
Hui CY Tang T Lin YY Chaudhury MK 《Langmuir : the ACS journal of surfaces and colloids》2004,20(14):6052-6064
A new cohesive zone model is developed in order to study the mechanisms of adhesive and cohesive failures of soft rubbery materials. The fracture energy is estimated here using a strategy similar to that of Lake and Thomas (LT) by considering the dissipation of stored elastic energy followed by the extension and relaxation of polymer chains. The current model, however, departs from that of LT in that the force needed to break an interfacial bond does not have a fixed value; instead, it depends on the thermal state of the system and the rate at which the force is transmitted to the bond. While the force required to rupture a chain is set by the rules of thermomechanically activated bond dissociation kinetics, extension of a polymer chain is modeled within both the linear and nonlinear models of chain elasticity. Closed form asymptotic solutions are obtained for the dependence of crack propagation speed on the energy release rate, which are valid in two regimes: (I) slow crack velocity or short relaxation time for bond dissociation; (II) fast crack velocity or long relaxation time for bond dissociation. The rate independent and the zero temperature limit of this theory correctly reduces to the fracture model of LT. Detailed comparisons are made with a previous work by Chaudhury et al. which carried out an approximate analysis of the same problem. 相似文献
177.
C-peptide radioimmunoassay (C-peptide RIA) is widely used in determination of pancreatic B-cell secretion activity.125I labeled TyrC-peptide is indispensable in C-peptide RIA kit. Herein we discuss a way of obtaining recombinant Tyr-C-peptide.
Arg32Tyr human proinsulin mutant (R32Y-proinsulin) gene was constructed by site-directed mutagenesis and overexpressed inEscherichia coli. Purified R32Y-proinsulin was converted to insulin and Tyr-C-peptide by trypsin and carboxypeptidase B codigestion. Tyr-C-peptide
was isolated through reverse-phase HPLC (RP-HPLC) and identified by C-peptide RIA and amino acid analysis. 相似文献
178.
179.
[reaction: see text] OSW saponins, featuring a 16beta,17alpha-dihydroxycholest-22-one aglycon and an acylated beta-D-xylopyranosyl-(1-->3)-alpha-L-arabinopyranosyl residue attached to the 16-hydroxyl group, have recently been discovered from a group of lily plants, which show potent antitumor activities with a novel mechanism of action. This paper describes an aldol approach to the stereoselective construction of the 16alpha,17alpha-dihydroxycholest-22-one structure from 16alpha-hydroxy-5-androsten-17-ones and propionates. Elaboration of the aldol adducts toward OSW-1, involving installation of the isoamyl ketone side chain, inversion of the 16-hydroxyl configuration, and selective protection of the C22-oxy function, has been explored and accomplished. In particular, the present route was found convenient for the synthesis of OSW saponin analogues with a C22-ester side chain. Thus, the 23-oxa-analogue of OSW-1 (40) was prepared starting from the industrial dehydroisoandrosterone (1) in a linear eight-step sequence and in 26% overall yield. Analogues with a variety of modified side chains were prepared, via aldol condensation with propionates of varying length, thiopropionate, and acetate (for preparation of 68-75) or via aminolysis of the 22,16-lactone 26 (for preparation of the 23-N-analogues). Cross metathesis (CM) reaction was also found feasible for modification at the final stage from C22-allyl ester 70. Valuable structure-activity relationships (SAR), together with the practical synthetic approach, have thus been provided to set a new stage for further studies on this new type of antitumor structures. 相似文献
180.
Using high‐resolution transmission electronic micrograph (HR‐TEM) observation, one can clearly see the pore geometry of the MCM‐41 and SBA‐15 mesoporous silicas to determine that their pore shapes are hexagonal and round, respectively. With the perpendicular orientations of the nanochannels to the electron beam, parallel line images of the (100) and (110) repeating spacings were observed. In the SBA‐15 mesoporous silicas, there are byproducts of the granular silica and disordered mesostructures, attributed to the weak hydrogen interactions between Pluronic 123 blockcopolymer and the silica species. There are also many different and significant +π disclination defects in SBA‐15 and MCM‐41 surfactant‐silica composites. The SBA‐15 with a thicker silica wall is more stable under irradiation by high‐energy electron beams compared to MCM‐41, which has thinner wall thickness. Some carbon nanostructure impurities were found in some carbon films on the metal grids. 相似文献