首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   458篇
  免费   27篇
化学   411篇
晶体学   2篇
力学   5篇
数学   11篇
物理学   56篇
  2023年   4篇
  2022年   3篇
  2021年   8篇
  2020年   12篇
  2019年   16篇
  2018年   10篇
  2017年   6篇
  2016年   12篇
  2015年   10篇
  2014年   24篇
  2013年   32篇
  2012年   30篇
  2011年   34篇
  2010年   15篇
  2009年   12篇
  2008年   37篇
  2007年   30篇
  2006年   25篇
  2005年   28篇
  2004年   23篇
  2003年   22篇
  2002年   22篇
  2001年   1篇
  2000年   3篇
  1999年   1篇
  1998年   2篇
  1997年   6篇
  1996年   9篇
  1995年   1篇
  1994年   3篇
  1993年   5篇
  1992年   1篇
  1991年   1篇
  1989年   1篇
  1985年   5篇
  1984年   6篇
  1983年   4篇
  1982年   5篇
  1981年   7篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1972年   1篇
排序方式: 共有485条查询结果,搜索用时 15 毫秒
391.
The theory of KM2O-Langevin equations for stochastic processes (or more generally, flows in inner product spaces) have been developed in view of applications to time series analysis (e.g., Okabe and Nakano, 1991; Okabe, 1999, 2000; Okabe and Matsuura, 2000). In Klimek et al. (2002) and Matsuura and Okabe (2001, 2003), we have investigated degenerate flows, which is important in the analysis of time series obtained from deterministic dynamical systems. As a continuation, we shall in this paper derive an efficient algorithm by which the minimum norm coefficients of KM2O-Langevin equations are explicitly obtained in degenerate cases. The obtained results have close relations to the calculations of conditional expectations such as nonlinear predictors of stochastic processes (Matsuura and Okabe, 2001). The method has also potential applications to financial mathematics.  相似文献   
392.
We developed a capillary zone electrophoresis method with indirect UV detection for determination of ammonium cations and alkali and alkaline earth metal cations in jellyfish. As the background electrolyte, a mixture of N-methylbenzylamine, citrate, and 18-crown-6 was used for the complete separation of all analyte cations. The limits of detection were 0.13 - 0.34 mg l(-1) at a signal-to-noise ratio of three. The values of the relative standard deviation of peak area were 3.2 - 4.9%. The proposed method successfully determined the above analyte cations in jellyfish for approximately 4 min.  相似文献   
393.
The dihydroxyisoleucine-, thiazoline- and dehydroamino acid-containing pentapeptide of the thiostrepton family of peptide antibiotics was synthesized, which featured the β-lactone opening by phenylselenylation, the vinylzinc addition to the chiral sulfinimine, the Wipf oxazoline-thiazoline conversion method and the oxidative syn-elimination of the phenylseleno group.  相似文献   
394.
Copper‐catalyzed enantioselective allyl–allyl coupling between allylboronates and either Z‐acyclic or cyclic allylic phosphates using a new chiral N‐heterocyclic carbene ligand, bearing a phenolic hydroxy, is reported. This reaction occurs with exceptional SN2′‐type regioselectivities and high enantioselectivities to deliver chiral 1,5‐diene derivatives with a tertiary stereogenic center at the allylic/homoallylic position.  相似文献   
395.
Heterometal arrays in molecular aggregations were obtained by the spontaneous and ultrasound‐induced gelation of organic liquids containing the chiral, clothespin‐shaped trans‐bis(salicylaldiminato) d8 transition‐metal complexes 1 . Heterometallic mixtures of complexes 1 a (Pd) and 1 b (Pt) underwent strict heterochiral aggregation entirely due to the organic shell structure of the clothespin shape, with no effect of the metal cores. This phenomenon provides an unprecedented means of generating highly controlled heterometallic arrangements such as alternating sequences [(+)‐Pd(?)‐Pt(+)‐Pd(?)‐Pt ??? ] as well as a variety of single metal‐enriched arrays (e.g., [(+)‐Pt(?)‐Pd(+)‐Pd(?)‐Pd(+)‐Pd(?)‐Pd ??? ] and [(+)‐Pd(?)‐Pt(+)‐Pt(?)‐Pt(+)‐Pt(?)‐Pt ??? ]) upon the introduction of an optically active masquerading unit with a different metal core in the heterochiral single‐metal sequence. The present method can be applied to form various new aggregates with optically active Pd and Pt units, to allow 1) tuning of the gelation ultrasound sensitivity based on the different hearing abilities of the metal units; 2) aggregation‐induced chirality transfer between heterometallic species; and 3) aggregation‐induced chirality enhancement. A mechanistic rationale is proposed for these molecular aggregations based on the molecular structures of the units and the morphologies of the aggregates.  相似文献   
396.
Potential effects of tea and its constituents on SARS-CoV-2 infection were assessed in vitro. Infectivity of SARS-CoV-2 was decreased to 1/100 to undetectable levels after a treatment with black tea, green tea, roasted green tea, or oolong tea for 1 min. An addition of (−) epigallocatechin gallate (EGCG) significantly inactivated SARS-CoV-2, while the same concentration of theasinensin A (TSA) and galloylated theaflavins including theaflavin 3,3′-di-O-gallate (TFDG) had more remarkable anti-viral activities. EGCG, TSA, and TFDG at 1 mM, 40 µM, and 60 µM, respectively, which are comparable to the concentrations of these compounds in tea beverages, significantly reduced infectivity of the virus, viral RNA replication in cells, and secondary virus production from the cells. EGCG, TSA, and TFDG significantly inhibited interaction between recombinant ACE2 and RBD of S protein. These results suggest potential usefulness of tea in prevention of person-to-person transmission of the novel coronavirus.  相似文献   
397.
Objectives: This study was to clarify changes in physical function and quality of life (QOL) for postoperative, and to examine the influence of the amount of physical activity on these variables. Methods: This study included 29 patients who underwent gastrointestinal cancer surgery. The QOL measurement was used to the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire for preoperative and 2nd and 4th postoperative weeks. Physical function measured knee extension strength, 4 m walk time, 5 times sit-to-stand test, and 6-minute walk for preoperative and 1st and 2nd postoperative weeks. The amount of physical activity score was based on METs-hours, which is estimated from cumulative physical activity. As basic characteristics were investigated cancer stage, comorbidities and complications, and operative. Statistical analysis was repeated measures analysis of variance was performed to observe postoperative changes in physical function and QOL. Furthermore, stepwise multiple regression analysis was used to the parameters of physical function and QOL affected by the physical activity score were investigated. Results: Physical function decreased postoperatively and generally improved 2nd postoperative week. Though scores on the QOL functional scales improved, some items did not improve sufficiently. Multiple regression analysis showed that physical activity score had an effect on constipation and emotion functioning. Conclusions: Improvement in symptom scales is not sufficient in a short period of time, and they need to be followed up by increasing the amount of physical activity and promoting instantaneous exercise.  相似文献   
398.
A novel vinyl monomer 2-{(3-acrylamidopropyl)dimethyl ammonio}ethyl 2′-isopropyl phosphate (AAPI) having phosphatidylcholine analogous moiety and a series of new acrylamide monomers containing two long alkyl chains [stearyloleylacrylamide (SOAAm), distearylacrylamide (SSAAm), and n-docosyloleylacrylamide (DOAAm)] were synthesized and characterized. The polyacrylamides ( Va, Vb , and Vc ) were obtained from radical copolymerization of SOAAm, SSAAm, or DOAAm with AAPI in the presence of AIBN as an initiator. The investigations of these copolymers were carried out by x-ray analysis method, DSC, TG-DTA, and viscosity measurement, respectively. The copolymer Vb containing distearyl groups as hydrophobic side chains shows a crystalline state at room temperature, and the ordered structure disappears at high temperature. The phase transition from a crystalline state to a liquid-crystalline state was observed obviously only for copolymer Vb . In addition, the II-A isotherms and A-T isobars of these copolymers were measured and their stable LB films were also prepared. © 1996 John Wiley & Sons, Inc.  相似文献   
399.
A series of new phospholipid analogous acrylamide monomers ( 4a–e ) containing long alkyl chains as hydrophobic groups and containing phosphatidylcholine analogues as hydrophilic group were synthesized in high yields. The homopolymerizations and copolymerization ( 4b with 4e ) were carried out in the presence of a radical initiator. The structures and thermal properties of these polymers were investigated by x-ray diffraction analysis, DSC, and polarizing microscopy measurements. It has been revealed that these homopolymers ( 5b, 5c , and 5d ) which bear saturated long-hydrocarbon chains in the side chains exhibited not only orderly stacked bilayer structures at room temperature but also clear liquid crystalline behavior within a wide temperature range. The viscosity behavior of all polymers was found similar to usual polyelectrolytes in porlar solvent. The polymers obtained were also characterized by 1H-NMR, IR, and melting point, respectively. © 1996 John Wiley & Sons, Inc.  相似文献   
400.
Absorption spectra of polycrystalline L-, D-, and DL-tartaric acid have been measured by terahertz time domain spectroscopy (THz-TDS). Different absorption bands are observed for DL-tartaric acid and its enantiomers (L- and D-tartaric acid). This result shows that the THz-TDS can be used for distinguishing between DL-tartaric acid and enantiomers (L- and D-tartaric acid). Moreover, partial least square (PLS) can be found to improve the quantitation of L-tartaric acid in L- and DL-tartaric acid mixture by THz-TDS.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号