全文获取类型
收费全文 | 943篇 |
免费 | 21篇 |
国内免费 | 2篇 |
专业分类
化学 | 597篇 |
晶体学 | 4篇 |
力学 | 28篇 |
数学 | 95篇 |
物理学 | 242篇 |
出版年
2021年 | 8篇 |
2020年 | 7篇 |
2016年 | 16篇 |
2015年 | 11篇 |
2014年 | 17篇 |
2013年 | 35篇 |
2012年 | 40篇 |
2011年 | 46篇 |
2010年 | 25篇 |
2009年 | 32篇 |
2008年 | 41篇 |
2007年 | 38篇 |
2006年 | 47篇 |
2005年 | 44篇 |
2004年 | 37篇 |
2003年 | 27篇 |
2002年 | 32篇 |
2001年 | 34篇 |
2000年 | 29篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 13篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 16篇 |
1979年 | 12篇 |
1978年 | 12篇 |
1977年 | 15篇 |
1976年 | 22篇 |
1975年 | 13篇 |
1974年 | 9篇 |
1973年 | 18篇 |
1972年 | 7篇 |
1970年 | 7篇 |
1969年 | 15篇 |
1967年 | 8篇 |
1966年 | 7篇 |
1962年 | 5篇 |
排序方式: 共有966条查询结果,搜索用时 15 毫秒
71.
Stephen Fox 《Entropy (Basel, Switzerland)》2021,23(2)
Active inference is a physics of life process theory of perception, action and learning that is applicable to natural and artificial agents. In this paper, active inference theory is related to different types of practice in social organization. Here, the term social organization is used to clarify that this paper does not encompass organization in biological systems. Rather, the paper addresses active inference in social organization that utilizes industrial engineering, quality management, and artificial intelligence alongside human intelligence. Social organization referred to in this paper can be in private companies, public institutions, other for-profit or not-for-profit organizations, and any combination of them. The relevance of active inference theory is explained in terms of variational free energy, prediction errors, generative models, and Markov blankets. Active inference theory is most relevant to the social organization of work that is highly repetitive. By contrast, there are more challenges involved in applying active inference theory for social organization of less repetitive endeavors such as one-of-a-kind projects. These challenges need to be addressed in order for active inference to provide a unifying framework for different types of social organization employing human and artificial intelligence. 相似文献
72.
Bryan S. Holtzman Eric T. Roberts Nicholas S. Caminiti Jacob A. Fox Madison B. Goodstein Staci A. Hill Zitong B. Jia Isabelle N.-M. Leibler Michael L. Martini Gina M. Mendolia Sarah B. Nodder Molly S. Costanza-Robinson Richard C. Bunt 《Tetrahedron letters》2017,58(5):432-436
A nucleophile crossover experiment was used to monitor the reversibility of nucleophilic addition of benzylamine to π-allylpalladium complexes. Dppe, dppp, dppb, and PHOX showed more crossover than PPh3 and dppm in both DMF and dichloromethane. Crossover was inhibited by the addition of DBU or Cs2CO3, but much less elimination to diene side products was observed with Cs2CO3. Analysis of percent crossover vs. percent reaction completion using the PHOX ligand revealed that with added DBU or Cs2CO3 crossover only began occurring after 100% completion had been reached. 相似文献
73.
A series of novel luminescent cyclometalated Au(III) neutral complexes of the type cis-[(N(∧)C)AuL] [N(∧)C = 2-phenylpyridine (ppy), L = 1,1'-biphenyl (1)] and cis-[(N(∧)C)AuL(2)] [N(∧)C = 2-phenylpyridine (ppy), L = C(6)H(5) (2), C(6)F(5) (3), C(6)H(4)-CF(3)-p (4), 2-C(4)H(3)S (5)]; [N(∧)C = 2-(2-thienyl)pyridine (thpy), L = C(6)H(5) (6), C(6)F(5) (7)]; [N(∧)C = 2-(5-methyl-2-thienyl)pyridine (5 m-thpy), L = C(6)F(5) (8)] were successfully synthesized. The X-ray crystal structures of all compounds except 3 have been determined. These complexes were found to show long-lived emission in solution at room temperature. The emission origins of the complexes have been tentatively assigned to be derived from triplet states predominantly bearing intraligand (IL) character with some perturbation from the metal center. Density functional theory (DFT) calculations were performed to evaluate the stability associated with the complexes and TD-DFT calculations to ascertain the nature of the excited state. Variation of the cyclometalated ligands in the complexes readily leads to the tuning of the nature of the lower energy emissive states. 相似文献
74.
Dibromonitrosyl(dihydrogen)rhenium(I) complexes [ReBr(2)(NO)(PR(3))(2)(η(2)-H(2))] (1; R = iPr, a; Cy, b) and Me(2)NH·BH(3) (DMAB) catalyze at either 90 °C or ambient temperature under 10 bar of H(2) the hydrogenation of various terminal and cyclic alkenes (1-hexene, 1-octene, cyclooctene, styrene, 1,5-cyclooctadiene, 1,7-octadiene, α-methylstyrene). Maximum turnover frequency (TOF) values of 3.6 × 10(4) h(-1) at 90 °C and 1.7 × 10(4) h(-1) at 23 °C were achieved in the hydrogenation of 1-hexene. The extraordinary catalytic performance of the 1/DMAB system is attributed to the formation of five-coordinate rhenium(I) hydride complexes [Re(Br)(H)(NO)(PR(3))(2)] (2; R = iPr, a; Cy, b) and the action of the Lewis acid BH(3) originating from DMAB. The related 2/BH(3)·THF catalytic system also exhibits under the same conditions high activity in the hydrogenation of various alkenes with a maximum turnover number (TON) of 1.2 × 10(4) and a maximum TOF of 4.0 × 10(4) h(-1). For the hydrogenations of 1-hexene with 2a and 2b, the effect of the strength of the boron Lewis acid was studied, the acidity being in the following order: BCl(3) > BH(3) > BEt(3) ≈ BF(3) > B(C(6)F(5))(3) > BPh(3) ? B(OMe)(3). The order in catalytic activity was found to be B(C(6)F(5))(3) > BEt(3) ≈ BH(3)·THF > BPh(3) ? BF(3)·OEt(2) > B(OMe)(3) ? BCl(3). The stability of the catalytic systems was checked via TON vs time plots, which revealed the boron Lewis acids to cause an approximate inverse order with the Lewis acid strength: BPh(3) > BEt(3) ≈ BH(3)·THF > B(C(6)F(5))(3). For the 2a/BPh(3) system a maximum TON of 3.1 × 10(4) and for the 2a/B(C(6)F(5))(3) system a maximum TOF of 5.6 × 10(4) h(-1) were obtained in the hydrogenation of 1-hexene. On the basis of kinetic isotope effect determinations, H(2)/D(2) scrambling, halide exchange experiments, Lewis acid variations, and isomerization of terminal alkenes, an Osborn-type catalytic cycle is proposed with olefin before H(2) addition. The active rhenium(I) monohydride species is assumed to be formed via reversible bromide abstraction with the "cocatalytic" Lewis acid. Homogeneity of the hydrogenations was tested with filtration and mercury poisoning experiments. These "rhenium(I) hydride/boron Lewis acid" systems demonstrate catalytic activities comparable to those of Wilkinson- or Schrock-Osborn-type hydrogenations accomplished with precious metal catalysts. 相似文献
75.
To study the interaction of the fac-[M(CO)(3)](+) moiety (M = (99m)Tc, (188)Re) with DNA bases, we reacted [M(OH(2))(3)(CO)(3)](+) with 9-methylguanine (9-MeG), guanosine (G), and 2-deoxyguanosine (2dG). Two bases bind to the metal center via the N7 atoms. X-ray structure analysis of [(99)Tc(CH(3)OH)(9-MeG)(2)(CO)(3)](+) (4) (monoclinic, I2/a, a = 28.7533(14) A, b = 8.0631(4) A, c = 32.3600(15) A, beta = 91.543(6) degrees, V = 7499.6(6) A(3), Z = 8) and [Re(OH(2))(9-MeG)(2)(CO)(3)](+) (7) (monoclinic, P2(1)/n, a = 12.2873(11) A, b = 16.0707(13) A, c = 14.1809(16) A, beta = 103.361(12) degrees, V = 2724.4(5) A(3), Z = 4) reveals that the two bases are in a head-to-tail (HT) orientation. Kinetic studies show that the rates of substitution of the purine bases are comparable to that of one of the active forms of cisplatin. The bis-substituted complexes are generally less stable than the platinum adducts, and metalation of the bases is reversible. 相似文献
76.
Addition of the carbene 1,3-dimesitylimidazol-2-ylidene (IMes) to a toluene solution of Ag[closo-CB(11)H(12)] results in the formation of the complex [(IMes)(2)Ag](2)[Ag(2)[closo-CB(11)H(12)](4)], the anionic component of which contains two silver(I) centers bridged by two carboranes in addition to one terminally bound carborane on each metal, in the solid-state. Comparison of the observed (11)B[(1)H] NMR chemical shifts of [(IMes)(2)Ag](2)[Ag(2)[closo-CB(11)H(12)](4)] or Ag[closo-CB(11)H(12)] with [NBu(4)][closo-CB(11)H(12)] in CD(2)Cl(2) demonstrates that the silver ion interacts significantly with the cage in solution. Theoretical investigations using the ab initio/GIAO/NMR method of [closo-CB(11)H(12)](-) and Na[closo-CB(11)H(12)] as model geometries for the silver salts support experimental evidence for these Ag...[BH] interactions in solution. 相似文献
77.
Individual members of a family of highly absorptive porphyrins bearing symmetrically positioned flexible side chains form discotic liquid crystalline mesophases at modest temperature ranges. The order of this phase is preserved when a thin layer of this self-assembled material is cooled to room temperature. Exposure of such films to visible excitation, with or without an externally applied electrical field, produces photovoltaic effects and/or charge trapping. The latter effect constitutes the basis for optoelectronic information storage. 相似文献
78.
Maize kernel hardness classification by near infrared (NIR) hyperspectral imaging and multivariate data analysis 总被引:1,自引:0,他引:1
The use of near infrared (NIR) hyperspectral imaging and hyperspectral image analysis for distinguishing between hard, intermediate and soft maize kernels from inbred lines was evaluated. NIR hyperspectral images of two sets (12 and 24 kernels) of whole maize kernels were acquired using a Spectral Dimensions MatrixNIR camera with a spectral range of 960-1662 nm and a sisuChema SWIR (short wave infrared) hyperspectral pushbroom imaging system with a spectral range of 1000-2498 nm. Exploratory principal component analysis (PCA) was used on absorbance images to remove background, bad pixels and shading. On the cleaned images, PCA could be used effectively to find histological classes including glassy (hard) and floury (soft) endosperm. PCA illustrated a distinct difference between glassy and floury endosperm along principal component (PC) three on the MatrixNIR and PC two on the sisuChema with two distinguishable clusters. Subsequently partial least squares discriminant analysis (PLS-DA) was applied to build a classification model. The PLS-DA model from the MatrixNIR image (12 kernels) resulted in root mean square error of prediction (RMSEP) value of 0.18. This was repeated on the MatrixNIR image of the 24 kernels which resulted in RMSEP of 0.18. The sisuChema image yielded RMSEP value of 0.29. The reproducible results obtained with the different data sets indicate that the method proposed in this paper has a real potential for future classification uses. 相似文献
79.
Copping R Gaunt AJ May I Sharrad CA Collison D Helliwell M Fox OD Jones CJ 《Chemical communications (Cambridge, England)》2006,(36):3788-3790
(NH4)14Na4[(Np3W4O15)(H2O)3(BiW9O33)3].62H2O (1) and (NH4)14.5Na3.5[(Np3W4O15)(H2O)3(SbW9O33)3].40.5H2O (2) each contain three neptunyl(v) moieties encapsulated within heteropolyoxotungstate frameworks in which axial {NpO2}+ oxygens form one face of a WO6 octahedron. 相似文献
80.
Godden TM Quilter JH Ramsay AJ Wu Y Brereton P Boyle SJ Luxmoore IJ Puebla-Nunez J Fox AM Skolnick MS 《Physical review letters》2012,108(1):017402
We demonstrate coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10-100?ps) dissociation of a spin-polarized electron-hole pair. Full control of the hole spin is achieved by combining coherent rotations about two axes: Larmor precession of the hole spin about an external Voigt geometry magnetic field, and rotation about the optical axis due to the geometric phase shift induced by a picosecond laser pulse resonant with the hole-trion transition. 相似文献