全文获取类型
收费全文 | 4318篇 |
免费 | 119篇 |
国内免费 | 10篇 |
专业分类
化学 | 2627篇 |
晶体学 | 43篇 |
力学 | 229篇 |
数学 | 610篇 |
物理学 | 938篇 |
出版年
2023年 | 58篇 |
2022年 | 64篇 |
2021年 | 142篇 |
2020年 | 121篇 |
2019年 | 91篇 |
2018年 | 160篇 |
2017年 | 197篇 |
2016年 | 190篇 |
2015年 | 181篇 |
2014年 | 170篇 |
2013年 | 227篇 |
2012年 | 295篇 |
2011年 | 317篇 |
2010年 | 312篇 |
2009年 | 303篇 |
2008年 | 226篇 |
2007年 | 239篇 |
2006年 | 251篇 |
2005年 | 170篇 |
2004年 | 93篇 |
2003年 | 94篇 |
2002年 | 97篇 |
2001年 | 116篇 |
2000年 | 61篇 |
1999年 | 82篇 |
1998年 | 45篇 |
1997年 | 24篇 |
1996年 | 17篇 |
1995年 | 3篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1985年 | 8篇 |
1984年 | 16篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1970年 | 2篇 |
排序方式: 共有4447条查询结果,搜索用时 15 毫秒
41.
Joongjai Panpranot Songphol Aungkapipattanachai Thanapon Sangvanich Pawin Boonyaporn Piyasan Praserthdam 《Reaction Kinetics and Catalysis Letters》2007,91(2):195-202
N2O pretreatment has shown to result in enhancement of the performance of fresh commercial Pd-Ag/α-Al2O3 catalyst during selective acetylene hydrogenation. However, it showed no effect for the used and regenerated catalysts probably
due to changes int he metal arrangement on the catalyst surface after regeneration. 相似文献
42.
Tirayut Vilaivan 《Tetrahedron letters》2006,47(38):6739-6742
A rate enhancement of tert-butoxycarbonylation of aromatic amines by Boc2O in alcohols compared to aprotic solvents was demonstrated. Kinetic analysis by NMR suggested that the reaction in CD3OD was faster than in CDCl3 by a factor of 70. Reactions between Boc2O and various aliphatic and aromatic amines in ethanol provided the N-Boc derivatives in good to excellent yields in short reaction times. 相似文献
43.
Ming-Liang Zhang Yu-Peng Tian Xuan-Jun Zhang Jie-Ying Wu Sheng-Yi Zhang Dong Wang Min-Hua Jiang Suchada Chantrapromm Hoong-Kun Fun 《Transition Metal Chemistry》2004,29(6):596-602
A new Schiff base ligand derived from S-benzyldithiocarbazate and 4-[N-hydroxy ethyl-N-(methyl)amino]benzaldehyde (HL, where H is a dissociable proton) and its NiII, CuII, ZnII and PdII complexes were prepared and fully characterized. The structures of HL and Ni(L)2 were determined by X-ray diffraction analysis, which revealed that the geometry of the NiII ion is square-planar with two equivalent Ni=N and Ni=S bonds, and that the two neighboring molecules in two layers have weak contact. The electronic spectra and solution fluorescence of the ligand and the complexes were studied, and the quantum yields of single-photon fluorescence for the compounds were determined. The compounds possess two-photon absorption (t.p.a.) character and the t.p.a. coefficient and t.p.a. cross-section were determined by the Z-scan technique. Especially, the Zn(L)2complex and the HL ligand exhibit intensive two-photon fluorescence (t.p.f.) at 800 nm laser pulses in the femtosecond regime. 相似文献
44.
In the present era, a major drawback of current anti-cancer drugs is the lack of satisfactory specificity towards tumor cells. Despite the presence of several therapies against cancer, tumor homing peptides are gaining importance as therapeutic agents. In this regard, the huge number of therapeutic peptides generated in recent years, demands the need to develop an effective and interpretable computational model for rapidly, effectively and automatically predicting tumor homing peptides. Therefore, a sequence-based approach referred herein as THPep has been developed to predict and analyze tumor homing peptides by using an interpretable random forest classifier in concomitant with amino acid composition, dipeptide composition and pseudo amino acid composition. An overall accuracy and Matthews correlation coefficient of 90.13% and 0.76, respectively, were achieved from the independent test set on an objective benchmark dataset. Upon comparison, it was found that THPep was superior to the existing method and holds high potential as a useful tool for predicting tumor homing peptides. For the convenience of experimental scientists, a web server for this proposed method is provided publicly at http://codes.bio/thpep/. 相似文献
45.
The interaction between precipitated silica and chloroprene rubber (CR) was investigated using a nuclear magnetic resonance (NMR) technique. The results reveal that the silanol groups on silica surface could chemically react with CR. Crosslinking of CR is therefore possible in the presence of silica at high temperature. The effects of silica and ethylene thiourea (ETU) loadings on properties of the silica-filled CR were thereafter investigated. With increasing silica loading, the compound viscosity increases considerably due to the dilution effect. As silica could act as a curative for CR, increasing silica loading results in both faster cure rate and increased crosslink density. The optimum tensile strength is found at approximately 30 phr of silica loading. The results also show that silica loading has little effect on most aging properties, except the relative modulus in which it increases rapidly with increasing silica loading due to the post curing effect. Similar to the effect of silica loading, the compound viscosity, cure rate and crosslink density are all increased with increasing ETU loading. The tensile strength is, on the other hand, slightly affected by ETU loading. Exception is found at high loading where the tensile strength drops noticeably. Interestingly, aging resistance of the vulcanizate is found to improve with the addition of ETU. Explanation is given by the hindrance capability of ETU to post curing. 相似文献
46.
P.?TangboriboonratEmail author D.?Polpanich T.?Suteewong K.?Sanguansap U.?Paiphansiri C.?Lerthititrakul 《Colloid and polymer science》2003,282(2):177-181
A non-uniform mesh structure, i.e. a dense network near the surface of peroxide-prevulcanised natural rubber latex particles, was observed under transmission electron microscopy. In the initial period of prevulcanisation, the swelling ratio of the latex sheet decreased with longer reaction time while an increase in crosslink density of rubber particles containing polystyrene, prepared using the phase transfer/bulk polymerisation process, was noticed. The modulus of the rubber sheet increased up to maximum crosslinking and thereafter decreased. After removal of proteins from the latex membrane layer, derived from protein-lipid originally existing at the rubber particle surface, could not be detected. The absence of proteins, which act as free radical scavengers, resulted in a rapid diffusion of alkoxy radicals into the rubber phase of deproteinised latex and, therefore, a uniform crosslink distribution inside each particle was obtained. 相似文献
47.
Watanapokasin Yuwadee Nuchfoang Siriporn Nilwarangkoon Sirinun Sarangbin Somsak Kakizono Toshihide 《Applied biochemistry and biotechnology》2002,98(1-9):301-309
A bacterial strain, SWU-4, capable of using benzothiophene (BT) as a sole carbon and energy source was isolated from a petroleum-contaminated
site in Thailand and identified by 16S rRNA gene sequence analysis to be in the genus of Mycobacterium. The strain was Gram-positive, nonspore former, and grew at 50° C. Colonies of the strain on nutrient agar were rod-shaped,
smooth with a convex surface, slightly mucoid, and yellow pigmented. The thermophilic Mycobacterium sp. strain SWU-4 rapidly degraded 2% (w/v) BT at 50°C. Interestingly, this strain was able to degrade a wide variety of organosulfur
compounds including thiophene, bromo(α)thiophene, and 3-methylthiophene in liquid minimum medium at 50°C, which will be beneficial
for industrial applications. 相似文献
48.
《中国化学快报》2020,31(4):988-991
Designing efficient electrocatalysts with low Pt loadings for hydrogen evolution reaction(HER) is urgently required for renewable and sustainable energy conversion.Here,we report a strategy that Pt nanoparticulates are spontaneously immobilized on porous MXene/MAX monolith as HER catalysts by utilizing the redox reaction between Ti_3C_2T_x MXene and [PtCl_4]~2 in H_2 PtCl_6 aqueous solution.By taking advantage of homogeneously distributed Pt nanoparticulates on highly electrically conductive porous Ti_3C_2T_x/Ti_3AlC_2 monolith,the as-prepared electrocatalysts show high catalytic performance for hydrogen evolution.Specifically,the binder-free electrocatalysts have Pt loadings as low as 8.9 μg/cm~2,with low overpotential of 43 mV at a curre nt density of 10 mA/cm~2 and low Tafel slope that three times lower than porous Ti_3C_2T_x/Ti_3AlC_2 without Pt loading.This strategy offers a new approach to constructing ultra-low Pt-loading HER catalysts on the basis of in situ redox reaction between noble metal ions and MXenes. 相似文献
49.
Sequential extraction of phosphorus in soil and sediment using a continuous-flow system 总被引:1,自引:0,他引:1
Distribution and mobility of phosphorus in soil and sediment are usually studied by sequential extraction. In the extraction procedure, a sample is treated with a series of reagents to distinguish the phases to which phosphorus is associated such as carbonate and iron or manganese oxides, etc. There have been a number of extraction schemes presented for phosphorus. At present, all of the existing schemes are carried out batchwise. Phosphorus contents derived from all sequences are operationally defined and depend on experimental conditions.An extraction procedure, which is a continuous-flow-based technique, was recently proposed by our group for metals in soils and sediments. The extraction is carried out in a closed chamber through which extractants are passed sequentially. In this paper, the system was investigated using the extraction scheme of Hieltjes and Lijklema to study distribution of phosphorus in three certified reference materials (CRMs). A number of fractions were collected for each reagent for subsequent colorimetric determination. The results are compared with those obtained from a batch extraction. The summation of phosphorus contents of all phases were compared with the certified values and with the values obtained from total digestion. These results have demonstrated that the continuous extraction system developed is also applicable for fractionation of phosphorus. Advantage and disadvantage are discussed. 相似文献
50.
《Tetrahedron: Asymmetry》2006,17(13):1990-1994
Optically active cis-fused octalin acetates have been prepared by esterification of cis-fused octalols using Candida antarctica lipase (CALB—Novozym 435). The enzyme efficiently resolved the racemic octalols by kinetic resolution, to afford cis-fused octalols and cis-fused octalin acetates with high enantiomeric excesses (up to >99%) and good yields (>40%). 相似文献