全文获取类型
收费全文 | 481篇 |
免费 | 13篇 |
国内免费 | 2篇 |
专业分类
化学 | 363篇 |
晶体学 | 7篇 |
力学 | 4篇 |
数学 | 57篇 |
物理学 | 65篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 3篇 |
2016年 | 8篇 |
2015年 | 11篇 |
2014年 | 12篇 |
2013年 | 27篇 |
2012年 | 39篇 |
2011年 | 43篇 |
2010年 | 23篇 |
2009年 | 22篇 |
2008年 | 41篇 |
2007年 | 30篇 |
2006年 | 23篇 |
2005年 | 29篇 |
2004年 | 26篇 |
2003年 | 25篇 |
2002年 | 24篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 3篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1977年 | 1篇 |
1971年 | 2篇 |
1960年 | 1篇 |
1948年 | 2篇 |
1944年 | 2篇 |
1942年 | 1篇 |
1941年 | 1篇 |
1938年 | 2篇 |
1937年 | 2篇 |
1936年 | 1篇 |
1935年 | 1篇 |
1934年 | 2篇 |
1931年 | 1篇 |
1929年 | 2篇 |
1928年 | 3篇 |
排序方式: 共有496条查询结果,搜索用时 15 毫秒
101.
Junichiro Sakamoto Akiko Imaizumi Yoshinori Sasaki Takashi Kamio Mamoru Wakoh Mika Otonari-Yamamoto Tsukasa Sano 《Magnetic resonance imaging》2014
Purpose
To evaluate the use of the intravoxel incoherent motion (IVIM) technique in half-Fourier single-shot turbo spin-echo (HASTE) diffusion-weighted imaging (DWI), and to compare its accuracy to that of apparent diffusion coefficient (ADC) to predict malignancy in head and neck tumors.Patients and methods
HASTE DW images of 33 patients with head and neck tumors (10 benign and 23 malignant) were evaluated. Using the IVIM technique, parameters (D, true diffusion coefficient; f, perfusion fraction; D*, pseudodiffusion coefficient) were calculated for each tumor. ADC values were measured over a range of b values from 0 to 1000 s/mm2. IVIM parameters and ADC values in benign and malignant tumors were compared using Student's t test, receiver operating characteristics (ROC) analysis, and multivariate logistic regression modeling.Results
Mean ADC and D values of malignant tumors were significantly lower than those of benign tumors (P < 0.05). Mean D* values of malignant tumors were significantly higher than those of benign tumors (P < 0.05). There was no significant difference in mean f values between malignant and benign tumors (P > 0.05). The technique of combining D and D* was the best for predicting malignancy; accuracy for this model was higher than that for ADC.Conclusions
The IVIM technique may be applied in HASTE DWI as a diagnostic tool to predict malignancy in head and neck masses. The use of D and D* in combination increases the diagnostic accuracy in comparison with ADC. 相似文献102.
Tsuji M Gomi S Maeda Y Matsunaga M Hikino S Uto K Tsuji T Kawazumi H 《Langmuir : the ACS journal of surfaces and colloids》2012,28(24):8845-8861
Rapid sphere-to-prism (STP) transformation of silver was studied in aqueous AgNO(3)/NaBH(4)/polyvinylpyrrolidone (PVP)/trisodium citrate (Na(3)CA)/H(2)O(2) solutions by monitoring time-dependent surface plasmon resonance (SPR) bands in the UV-vis region, by examining transmission electron microscopic (TEM) images, and by analyzing emitted gases during fast reaction. Roles of PVP, Na(3)CA, and H(2)O(2) were studied without addition of a reagent, with different timing of each reagent's addition, and with addition of H(2)O(2) to mixtures of spheres and prisms. Results show that prisms can be prepared without addition of PVP, although it is useful to synthesize smaller monodispersed prisms. A new important role of citrate found in this study, besides a known role as a protecting agent of {111} facets of plates, is an assistive agent for shape-selective oxidative etching of Ag nanoparticles by H(2)O(2). The covering of Ag nanoparticles with carboxylate groups is necessary to initiate rapid STP transformation by premixing citrate before H(2)O(2) addition. Based on our data, rapid prism formation starts from the consumption of spherical Ag particles because of shape-selective oxidative etching by H(2)O(2). Oxidative etching of spherical particles by H(2)O(2) is faster than that of prisms. Therefore, spherical particles are selectively etched and dissolved, leaving only seeds of prisms to grow into triangular prisms. When pentagonal Ag nanorods and a mixture of cubes and bipyramids were used as sources of prisms, rod-to-prism (RTP), cube-to-prism (CTP), and bipyramid-to-prism (BTP) transformations were observed in Ag nanocrystals/NaBH(4)/PVP/Na(3)CA/H(2)O(2) solutions. Shape-selective oxidative etching of rods was confirmed using flag-type Ag nanostructures consisting of a triangular plate and a side rod. These data provide useful information for the size-controlled synthesis of triangular Ag prisms, from various Ag nanostructures and using a chemical reduction method, having surface plasmon resonance (SPR) bands at a desired wavelength. 相似文献
103.
Inka Saarikoski Fatima Joki-Korpela Mika Suvanto Tuula T. Pakkanen Tapani A. Pakkanen 《Surface science》2012,606(1-2):91-98
New approaches to the fabrication of microstructures of special shape were developed for polymers. Unusual superhydrophobic surface structures were achieved with the use of flexible polymers and hierarchical molds.Flexible polyurethane?acrylate coatings were patterned with microstructures with use of microstructured aluminum mold in a controlled UV-curing process. Electron microscope images of the UV-cured coatings on polymethylmethacrylate (PMMA) substrates revealed micropillars that were significantly higher than the corresponding depressions of the mold (even 47 vs. 35 μm). The elongation was achieved by detaching the mold from the flexible, partially cured acrylate surface and then further curing the separated microstructure. The modified acrylate surface is superhydrophobic with a water contact angle of 156° and sliding angle of < 10°.Acrylic thermoplastic elastomers (TPE) were patterned with micro?nanostructured aluminum oxide molds through injection molding. The hierarchical surface of the elastomer showed elongated micropillars (57 μm) with nail-head tops covered with nanograss. Comparison with a reference microstructure of the same material (35 μm) indicated that the nanopores of the micro?nanomold assisted the formation of the nail-shaped micropillars. The elasticity of the TPE materials evidently plays a role in the elongation because similar elongation has not been found in hierarchically structured thermoplastic surfaces. The hierarchical micronail structure supports a high water contact angle (164°), representing an increase of 88° relative to the smooth TPE surface. The sliding angle was close to zero degrees, indicating the Cassie–Baxter state. 相似文献
104.
Kopreski RP Briggs JB Lin W Jazdzyk M Miller GP 《The Journal of organic chemistry》2012,77(3):1308-1315
Several o-quinodimethane adducts of [60]fullerene were synthesized and their intramolecular aryl CH-fullerene π interactions were studied using variable temperature-NMR (VT-NMR). Evaluation of the rate constants associated with the first-order transition states for cyclohexene boat-to-boat inversions enables quantification of ΔG(?) values for each inversion. A comparison between two constitutional isomers, only one of which is capable of intramolecular CH-π interactions, provides a lower limit of 0.95 kcal/mol for each aryl CH-fullerene π interaction. 相似文献
105.
Hira D Okuda T Ichihashi M Mizutani A Ishizeki K Okada T Okamoto H 《Chemical & pharmaceutical bulletin》2012,60(3):341-347
The aim of this study was to reveal the relationship between human inspiratory flow patterns and the concomitant drops in pressure in different inhalation devices, and the influence of the devices on inhalation performance. As a model formulation for inhalers, a physically mixed dry powder composed of salbutamol sulfate and coarse lactose monohydrate was selected. The drops in pressure at 28.3?L/min of three inhalation devices, Single-type, Dual-type, and Reverse-type, was 1.0, 5.1, and 8.7?kPa, respectively. Measurements of human inspiratory patterns revealed that although the least resistant device (Single) had large inter- and intra-individual variation of peak flow rate (PFR), the coefficients of variation of PFR of the three devices were almost the same. In tests with a human inspiratory flow simulator in vitro, inhalation performance was higher, but the variation in inhalation performance in the range of human flow patterns was wider, for the more resistant device. To minimize the intra- and inter-individual variation in inhalation performance for the model formulation in this study, a formulation design that allows active pharmaceutical ingredient to detach from the carrier with a lower inhalation flow rate is needed. 相似文献
106.
Given a vertex v of a graph G the second order degree of v denoted as d 2(v) is defined as the number of vertices at distance 2 from v.In this paper we address the following question:What are the sufficient conditions for a graph to have a vertex v such that d2(v) ≥ d(v),where d(v) denotes the degree of v? Among other results,every graph of minimum degree exactly 2,except four graphs,is shown to have a vertex of second order degree as large as its own degree.Moreover,every K-4-free graph or every maximal planar graph is shown to have a vertex v such that d2(v) ≥ d(v).Other sufficient conditions on graphs for guaranteeing this property are also proved. 相似文献
107.
Joanna Józefowska Marek Mika Rafał Różycki Grzegorz Waligóra Jan Weglarz 《Mathematical Methods of Operations Research》2000,52(3):489-499
In this paper a discrete-continuous project scheduling problem is considered. In this problem activities simultaneously require
discrete and continuous resources. The processing rate of each activity depends on the amount of the continuous resource allotted
to this activity at a time. All the resources are renewable ones. The activities are nonpreemtable and the objective is to
minimize the makespan. Discretization of this problem leading to a classical (i.e. discrete) project scheduling problem in
the multi-mode version is presented. A simulated annealing (SA) approach to solving this problem is described and tested computationally
in two versions: with and without finding an optimal continuous resource allocation for the final schedule. In the former
case a nonlinear solver is used for solving a corresponding convex programming problem. The results are compared with the
results obtained using SA for the discrete-continuous project scheduling problem where the nonlinear solver is used for exact
solving the continuous part in each iteration. The results of a computational experiment are analyzed and some conclusions
are included. 相似文献
108.
Atsushi Sudo Kyohei Nishiyama Mika Morimoto Shinichiro Yamamoto 《Journal of polymer science. Part A, Polymer chemistry》2020,58(14):1973-1981
Dimethacrylates with rigid adamantane-like cores were synthesized from myo-inositol orthoester via a sequence of (a) acylation or silylation of the equatorially oriented hydroxyl group, followed by (b) attachment of methacrylate groups on the axially oriented hydroxyl groups. The radical homopolymerization of these compounds proceeded via cyclopolymerization without crosslinking, as the two axially oriented methacrylate groups were fixed in close proximity with each other. The dimethacrylates underwent radical copolymerization with methyl methacrylate (MMA) to afford the corresponding polymethacrylates, exhibiting high glass transition temperatures (Tg), due to the introduction of the rigid orthoester moieties originating from the monomers and the macrocyclic structures formed via intramolecular cyclization of the two methacrylate groups of the monomers. The polymers obtained by polymerization of the dimethacrylate bearing a silylated hydroxyl group served as precursors of hydroxyl-bearing polymers, which also exhibited high Tg due to the formation of a hydrogen bonding network between the hydroxyl groups. 相似文献
109.
Wilfred F. van Gunsteren Dirk Bakowies Riccardo Baron Indira Chandrasekhar Markus Christen Xavier Daura Peter Gee Daan P. Geerke Alice Glttli Philippe H. Hünenberger Mika A. Kastenholz Chris Oostenbrink Merijn Schenk Daniel Trzesniak Nico F. A. van der Vegt Haibo B. Yu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2006,118(25):4168-4198
110.
Toshihide Tsukatani Hajime Katano Hirosuke Tatsumi Mika Deguchi Naoki Hirayama 《Analytical sciences》2006,22(2):199-200
The tetraoctylammonium cation forms water-immiscible room temperature ionic liquids with dodecylsulfate and dodecylbenzenesulfonate anions. The ionic liquids are halogen-free and can be considered environmentally friendly solvents. At 25 degrees C, the solubilities of water in tetraoctylammonium dodecylsulfate and tetraoctylammonium dodecylbenzenesulfonate were 2.8 and 4.0%, respectively, and the densities were 0.92 and 0.93 g cm(-3). The ionic liquids formed stable interfaces with water at 25 degrees C. The possible use of tetraoctylammonium dodecylsulfate as chelate extraction solvent was evaluated by using 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione as extractant. Tetraoctylammonium dodecylsulfate showed high extraction performance for divalent transition metal cations. 相似文献