全文获取类型
收费全文 | 6697篇 |
免费 | 178篇 |
国内免费 | 46篇 |
专业分类
化学 | 4893篇 |
晶体学 | 73篇 |
力学 | 158篇 |
数学 | 714篇 |
物理学 | 1083篇 |
出版年
2023年 | 27篇 |
2022年 | 50篇 |
2021年 | 51篇 |
2020年 | 76篇 |
2019年 | 99篇 |
2018年 | 71篇 |
2017年 | 51篇 |
2016年 | 143篇 |
2015年 | 119篇 |
2014年 | 128篇 |
2013年 | 305篇 |
2012年 | 401篇 |
2011年 | 489篇 |
2010年 | 239篇 |
2009年 | 205篇 |
2008年 | 409篇 |
2007年 | 464篇 |
2006年 | 470篇 |
2005年 | 459篇 |
2004年 | 417篇 |
2003年 | 335篇 |
2002年 | 289篇 |
2001年 | 102篇 |
2000年 | 81篇 |
1999年 | 77篇 |
1998年 | 81篇 |
1997年 | 59篇 |
1996年 | 86篇 |
1995年 | 63篇 |
1994年 | 61篇 |
1993年 | 50篇 |
1992年 | 43篇 |
1991年 | 54篇 |
1990年 | 46篇 |
1989年 | 41篇 |
1988年 | 40篇 |
1987年 | 33篇 |
1986年 | 35篇 |
1985年 | 55篇 |
1984年 | 58篇 |
1983年 | 49篇 |
1982年 | 53篇 |
1981年 | 56篇 |
1980年 | 56篇 |
1979年 | 49篇 |
1978年 | 55篇 |
1977年 | 48篇 |
1976年 | 47篇 |
1975年 | 28篇 |
1974年 | 26篇 |
排序方式: 共有6921条查询结果,搜索用时 15 毫秒
91.
We present the first recording of the high-resolution spectrum of an induced chiral dimer. Three conformers of the induced chiral dimers of ethanol have been observed using a pulsed molecular-beam Fourier transform microwave spectrometer. The rotational constants of the normal isotopomers of the three species have been determined to be (a) A=5113.826(5), B=1329.7214(4), and C=1257.5151(3) MHz, (b) A=5086.459(5), B=1316.6508(4), and C=1243.6329(4) MHz, and (c) A=4851.608(5), B=1369.7558(6), and C=1243.4184(4) MHz. The observed species have been assigned to calculated structures via Kraitchman double substitution analyses and ab initio calculations. The Kraitchman analyses and the fitted centrifugal distortion parameters suggest that the deuterium bond is significantly stronger than the hydrogen bond in the dimers of ethanol. 相似文献
92.
Englert BC Scholz S Leech PJ Srinivasarao M Bunz UH 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(3):995-1000
We describe the synthesis of two cyclobutadiene(cyclopentadienyl)cobalt-containing poly(p-phenylene ethynylene)s (PPEs) and their use as precursors for stable ceramic surface coatings. Organometallic PPEs were shaped into hexagonally ordered assemblies by using the breath-figure method. Such breath figures can be washed away with an appropriate solvent. Upon pyrolysis at 500 degrees C under either nitrogen or air, the bubble arrays persist as ceramics and are insoluble in organic solvents or water. The formed pyrolyzed bubble arrays were analyzed by optical and scanning electron microscopy, as well as energy dispersive X-ray microanalysis (EDX). The composition of the ceramic materials is discussed based on EDX and IR data. 相似文献
93.
Wehrenberg BL Yu D Ma J Guyot-Sionnest P 《The journal of physical chemistry. B》2005,109(43):20192-20199
Conduction in thin films of PbSe nanocrystals doped by electrochemical gating has been studied. Charging the film, with either electrons or holes, increases the conductance by orders of magnitude. The electrons in the 1S(e) state of nanocrystals in these films have a mobility as high as 5.0 x 10(-3) cm(2) V(-1) s(-1). Electrons in the 1P(e) state were found to have a differential mobility up to 3-5 times greater than the electrons in the 1S(e) state, and a mobility minima was found corresponding to the complete filling of the 1S(e) state. The temperature and electric field dependence of conductance in the film, measured between 4.3 and 135 K, were both well described by a variable range hopping model. 相似文献
94.
Brian F.G. Johnson Jack Lewis William J.H Nelson Maria D. Vargas Dario Braga Mary McPartlin 《Journal of organometallic chemistry》1983,246(2):c69-c73
The high nuclearity mixed metal cluster monoanions [Os10C(CO)24Cu(NCMe)]? (I) and [Os10C(CO)24AuPPh3]? (II) have been obtained by reaction of the carbido-dianion [Os10C(CO)24]2? (III) with one equivalent of [Cu(NCMe)4] [BF4] and Ph3PAuCl, respectively, in CH2Cl2. X-ray analysis of the [PPh3Me]+ salts of I and II show that the Cu and Au ligands have added to capping tetrahedra of the dianion III in μ3- and μ2-bridging positions, respectively. 相似文献
95.
Brian Dickens 《Polymer Degradation and Stability》1980,2(4):249-268
Factor-jump thermogravimetry has been used to study the activation energy of polystyrene degrading in a vacuum, in N2 flowing at 4 mm/s and in mixtures. The results show the activation energy to be 44·9 ± 0·2 kcal/mole (188 ± 0·8 kJ/mole) for degradation above 350°C in vacuum or in flowing N2. This agrees well with work reported in 1949 by Jellinek7 but with few results reported subsequently.The apparent activation energy for polystyrene losing weight above 280°C in an atmosphere of abundant O2 is 21·5 ± 0·2 kcal/mole (90·2 ± 0·8 kJ/mole). In all cases where O2 was deliberately introduced (partial pressures >4 mm Hg), the sample degraded to a black tar and the activation energy was ≤30 kcal/mole, depending on the amount of oxygen present and on the thermal history of the sample. 相似文献
96.
Leslie D. Field Adrian V. George Trevor W. Hambley Peter Turner Brian J. Wallace 《Journal of organometallic chemistry》1997,540(1-2):95-99
Reaction of cis-[FeH2(dmpe)2](1) with diphenylbutadiyne results in an insertion into both of the iron-hydride bonds to form an iron metallocycle. Spectroscopic and crystallographic data of [Fe(PhHCC2CHPh)(dmpe)2] (3) show 1,4-diphenylbutatriene is symmetrically bound to the metal via the central double bond. The reaction to form the metallocyclic complex is greatly accelerated by application of external pressure. A 41% yield of (3) is isolated after two days at atmospheric pressure or after approximately 75 min at 800MPa. 相似文献
97.
The importance of applying a variety of experimental techniques to unravel the nature of solvated electrons is emphasized. Compton profiles are evaluated for these species from a range of models. Some comparisons are made with positron annihilation studies. 相似文献
98.
The solubilization of styrene by poly(butyl methacrylate-g-methoxypoly(ethylene glycol)) and poly(methyl methacrylate-g-methoxypoly(ethylene glycol)) graft copolymers has been examined. From turbidity measurements the solubility limit of the monomer in the micelles was obtained and the distribution coefficients were evaluated. Dynamic light scattering revealed that below the solubility limit, solubilization leads to a slight increase in micelle size, while above the solubility limit, there is a dramatic increase in particle size and turbidity as oil-in-water emulsions are formed through coalescence of monomer-swollen micelles. Polymerizations carried out below the solubility limit using the graft copolymer micelles as templates resembled microemulsion polymerizations in nature and led to very fine sterically stabilized polystyrene latex particles. Through careful control of the monomer concentration and the polymerization temperature it was possible to obtain spherical nanosize latex particles with similar size to those of the micelle precursors (10 nm) up to 11% monomer by weight. Polymerizations above the solubility limit, on the other hand, showed similarities with emulsion polymerizations and resulted in larger particles with higher polydispersity. 相似文献
99.
Photosensitizer biodistribution change inside tissue is one of the dominant factors in photodynamic therapy efficacy. In this study, the pharmacokinetics of a benzoporphyrin derivative (BPD), delivered in verteporfin for injection formulation, have been quantified in the rat Dunning prostate tumor MAT-LyLu model, using both subcutaneous and orthotopic sites. Blood plasma sampling indicated that BPD had a bi-exponential metabolic lifetime in vivo, with the two lifetimes being 9.6 min and 8.3 h. The spatial distributions in the tumor were quantified as a function of distance from the perfused blood vessels, using fluorescence histologic images of the tumor. A fluorescent vascular marker was used to obtain locations and shapes of perfused capillaries at a wavelength of emission different from that of BPD and to allow colocalized images to be acquired of vessel and BPD locations. Using the BPD fluorescence images obtained 15 min after intravenous administration, a forward finite-element solution to the diffusion equation was used to predict the drug distribution by matching the fluorescence intensity images observed microscopically. An inverse solver was used to minimize the root mean square error between the image of simulated diffusion and the experimental image, resulting in estimation of the diffusion coefficient of BPD in the tumor models. Effective diffusion coefficients were 0.88 and 1.59 microm2/s for the subcutaneous and orthotopically grown tumors, respectively, indicating that orthotopic tumors have significantly higher vascular extravasation rates as compared with subcutaneous tumors. This analysis supports the hypothesis that leakage rates of the photosensitizer vary considerably. Thus, although varying the time between injection and optical irradiation may be used to vary the targeting between vascular and less vascular areas, the precise time of treatment will depend on the nature of the permeability of the vasculature in the tissue being treated. 相似文献
100.
Light Dosimetry for Intraperitoneal Photodynamic Therapy in a Murine Xenograft Model of Human Epithelial Ovarian Carcinoma 总被引:1,自引:0,他引:1
Lothar Lilge Kelly Molpus Tayyaba Hasan Brian C. Wilson 《Photochemistry and photobiology》1998,68(3):281-288
Few studies have been published to date measuring spatially resolved fluence rates in complex tissue geometries. Here the light distributions of three different intraperitoneal light delivery geometries in a murine ovarian cancer model were investigated to assess their influence on the tumorcidal efficacy of photodynamic therapy (PDT). In vivo fluence rate measurements in the peritoneal cavities of mice, with the light intensity being mapped in three transverse planes, were performed using fiber-optic detectors. Three different source fiber designs and placements were tested for their ability to provide uniform irradiation of the peritoneal cavity. The biological response to a PDT protocol comprising three separate treatments administered at 72 h intervals, each consisting of a 0.25 mg kg intraperitoneal injection of benzopor-phyrin derivative-mono acid ring A followed 90 min later by delivery of 15 J of 690 nm light, was measured. The tissue response was evaluated by measuring the number of remaining visible lesions and the total residual tumor mass. Fluence rate measurements showed large variations in the fluence rate distribution for similar intended treatments. The most uniform and reproducible illumination was achieved using two 18 mm long cylindrical emitting optical fibers. The biological response was comparable to that produced when a flat-cleaved end optical fiber is used to illuminate the four quadrants of the abdomen sequentially. While a good reproducibility in tumor induction in this animal model exists, no correlation was found between the fluence rate distribution measured in one group of animals and the biological response in a separate group of similarly treated animals. Due to the large intra-animal variability in fluence rate distribution, representative fluence rate mapping in complex tissue geometries is of limited value when applied to an individual PDT treatment. Thus, surveillance of the fluence rate during individual treatments will be required for acceptable PDT dosimetry. To improve the versatility of this particular animal model for PDT research, a large number of extended sources are required to increase uniformity of the illumination in order to reduce unwanted cytotoxic side effects resulting from foci of high fluence rates. In this way, subsequent increase of the total energy delivered to the tumor may be possible. 相似文献