全文获取类型
收费全文 | 2626篇 |
免费 | 91篇 |
国内免费 | 6篇 |
专业分类
化学 | 1620篇 |
晶体学 | 123篇 |
力学 | 85篇 |
数学 | 253篇 |
物理学 | 642篇 |
出版年
2023年 | 17篇 |
2022年 | 41篇 |
2021年 | 46篇 |
2020年 | 41篇 |
2019年 | 49篇 |
2018年 | 54篇 |
2017年 | 45篇 |
2016年 | 78篇 |
2015年 | 75篇 |
2014年 | 82篇 |
2013年 | 232篇 |
2012年 | 136篇 |
2011年 | 132篇 |
2010年 | 107篇 |
2009年 | 99篇 |
2008年 | 121篇 |
2007年 | 109篇 |
2006年 | 84篇 |
2005年 | 91篇 |
2004年 | 76篇 |
2003年 | 62篇 |
2002年 | 72篇 |
2001年 | 33篇 |
2000年 | 24篇 |
1999年 | 27篇 |
1998年 | 16篇 |
1997年 | 21篇 |
1996年 | 32篇 |
1995年 | 27篇 |
1994年 | 18篇 |
1993年 | 31篇 |
1992年 | 30篇 |
1991年 | 32篇 |
1990年 | 28篇 |
1989年 | 37篇 |
1988年 | 26篇 |
1987年 | 24篇 |
1986年 | 31篇 |
1985年 | 30篇 |
1984年 | 34篇 |
1983年 | 31篇 |
1982年 | 26篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 25篇 |
1978年 | 15篇 |
1977年 | 34篇 |
1976年 | 16篇 |
1975年 | 22篇 |
1974年 | 20篇 |
排序方式: 共有2723条查询结果,搜索用时 15 毫秒
101.
Cellulose nanofibre aspect ratio controls the properties of sheets made from nanofibres and processing conditions, but aspect ratio is very difficult to measure. In this paper, aspect ratio was estimated from the gel point of a cellulose nanofibre suspension, the solids concentration at which the transition from a dilute to a semi-dilute suspension occurs. Four batches of cellulose nanofibres were tested. Two were produced from softwood fibres using ball milling. Commercially produced microfibrillated cellulose material was also used, both in as supplied form and after removal of the larger fibres by filtering. The average diameter measured from SEM images of fibres ranged from 33 to 73 nm. One sample was too heavily treated and an average dimension could not be measured. The gel-point was measured both from the height of a layer of cellulose nanofibres sedimented from a dilute suspension or from the lowest solids concentration at which a yield stress could be measured using a vane rheometer. The two methods were closely in agreement for all samples. Aspect ratio was then calculated using either the effective medium (EMT) or crowding number (CN) theories. Aspect ratio calculated with an assumed fibre density of 1,500 kg/m3, using the CN theory ranged from 155 to 60. Use of the EMT theory reduced the calculated aspect ratio by between 11 and 23 %. Reducing the assumed density in suspension from 1,500 to 1,166 kg/m3 reduced the calculated aspect ratio by 12–14 %. The heavily treated sample had by far the lowest aspect ratio. 相似文献
102.
David S. Maxwell Duoli SunZhenghong Peng Diana V. MartinBasvoju A. Bhanu Prasad William G. Bornmann 《Tetrahedron letters》2013
We report the synthesis of a macrocycle utilizing a novel framework of standard amino acids in combination with subunits that we have named as Linked Amino Acid Mimetics (LAAMs). Macrocycles based on the LAAM concept provide both a peptide targeting region and two independently variable functional regions. In the prototype structure, the commonly known Arg-Gly-Asp (RGD) sequence was used for the targeting region. The functional regions contain a phenyl group, and the linkage was formed via a Ring-Closing Metathesis (RCM) reaction. 相似文献
103.
Dr. Manivannan Ethirajan Dr. Ping Chen Dr. Tymish Y. Ohulchanskyy Dr. Lalit N. Goswami Dr. Anurag Gupta Dr. Avinash Srivatsan Dr. Mahabeer P. Dobhal Joseph R. Missert Prof. Paras N. Prasad Prof. Karl M. Kadish Prof. Ravindra K. Pandey 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(21):6670-6684
We report herein a simple and efficient approach to the synthesis of a variety of meso‐substituted purpurinimides. The reaction of meso ‐ substituted purpurinimide with N‐bromosuccinimide regioselectively introduced a bromo functionality at the 20‐position, which on further reaction with a variety of boronic acids under Suzuki reaction conditions yielded the corresponding meso‐substituted analogues. Interestingly, the free base and the metalated analogues showed remarkable differences in photosensitizing efficacy (PDT) and tumor‐imaging ability. For example, the free‐base conjugate showed significant in vitro PDT efficacy, but limited tumor avidity in mice bearing tumors, whereas the corresponding NiII derivative did not produce any cell kill, but showed excellent tumor‐imaging ability at a dose of 0.3 μmol kg?1 at 24, 48, and 72 h post‐injection. The limited PDT efficacy of the NiII analogue could be due to its inability to produce singlet oxygen, a key cytotoxic agent required for cell kill in PDT. Based on electrochemical and spectroelectrochemical data in DMSO, the first one‐electron oxidation (0.52 V vs. SCE) and the first one‐electron reduction (?0.57–0.67 V vs. SCE) of both the free base and the corresponding NiII conjugates are centered on the cyanine dye, whereas the second one‐electron reduction (?0.81 V vs. SCE) of the two conjugates is assigned to the purpurinimide part of the molecule. Reduction of the cyanine dye unit is facile and occurs prior to reduction of the purpurinimide group, which suggests that the cyanine dye unit as an oxidant could be the driving force for quenching of the excited triplet state of the molecules. An interaction between the cyanine dye and the purpurinimide group is clearly observed in the free‐base conjugate, which compares with a negligible interaction between the two functional groups in the NiII conjugate. As a result, the larger HOMO–LUMO gap of the free‐base conjugate and the corresponding smaller quenching constant is a reason to decrease the intramolecular quenching process and increase the production of singlet oxygen to some degree. 相似文献
104.
Dr. Bishnu Prasad Bastakoti Prof. Kevin C.‐W. Wu Dr. Masamichi Inoue Prof. Shin‐ichi Yusa Prof. Kenichi Nakashima Prof. Yusuke Yamauchi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(15):4812-4817
We have developed core‐shell‐corona‐type polymeric micelles that can integrate multiple functions in one system, including the capability of accommodating hydrophobic dyes into core and hydrophilic drug into the shell, as well as pH‐triggered drug‐release. The neutral and hydrophilic corona sterically stabilizes the multifunctional polymeric micelles in aqueous solution. The mineralization of calcium phosphate (CaP) on the PAA domain not only enhances the diagnostic efficacy of organic dyes, but also works as a diffusion barrier for the controlled release. 相似文献
105.
Synthesis of new benzoxazaphosphinine/benzoxazaphosphole/diazaphosphaphenalene 2‐sulfides were accomplished by the reaction of Lawesson's reagent (LR) with 4‐bromo‐2‐[(phenylamino) methyl]phenol (1a), 4‐bromo‐2‐[(4‐chloro/bromo/methoxy/methylphenyl‐amino)methyl]phenol (1b–e), 4‐bromo‐2‐[(benzylamino)methyl]phenol (1f), 2‐amino‐4‐chlorophenol (2a)/2‐amino‐4‐methylphenol (2b), 1,8‐diaminonaphthalene (3) respectively in anhydrous toluene. Products 4a–f, 5a–b and 6 were characterized by IR, 1H, 13C, 31P NMR and Mass spectra. 相似文献
106.
M. A. Sridhar N. K. Lokanath J. Shashidhara Prasad C. V. Yelammagad S. K. Varshney 《Liquid crystals》2013,40(1):45-49
The title compound, cholesteryl 6[4-(4-pentyloxyphenylethynyl)phenoxy]hexanoate (DMT5), a cholesterol-based dimesogen, crystallizes in the monoclinic space group P21 with the parameters a = 9.812(3) Å, b = 9.713(2) Å, c = 24.179(2) Å, β = 92.48(1)° V = 2302.2(9) Å3, Z = 2, F000 = 836, λ(MoKα) = 0.71069 Å, μ = 0.067 cm 1, final R = 0.0772. The structure has intermolecular hydrogen bonds; it is stabilized by the presence of intermolecular contacts whose spacings are less than van der Waals' radii. 相似文献
107.
S. Krishna Prasad R. Shashidhar B. R. Ratna B. K. Sadashiva G. Heppke S. Pfeiffer 《Liquid crystals》2013,40(1):111-116
Abstract We present here precise measurements of the layer spacing for two compounds, viz. 4-n-heptyloxyphenyl-4′-(4″-cyanobenzoyloxy)benzoate and 4-n-undecyloxyphenyl-4′-(4″-cyanobenzyloxy)benzoate. Contrary to earlier reports our data do not show any jump in the layer spacing at the expected Ad-A2 transition temperature, showing thereby that for both compounds A2 evolves continuously from the Ad phase. 相似文献
108.
Transparent glasses of various compositions in the system (100−x)Li2B4O7−x(SrO-Bi2O3-Nb2O5) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi2Nb2O9 phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple-Didomemenico and Gladstone-Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active. 相似文献
109.
For the first time, we report the acoustic wave detection of chemical species being transported in a capillary tube to a region where acoustic coupling occurs. The measured parameter was a change in phase, which was originally only attributed to a change in solution density as the analyte passed by the detection region. Accordingly, we report the detection of change in phase as various chemical species (e.g. Cy5 dye, Cy5-derivatized glycine and underivatized glycine) were introduced into and migrated along a capillary tube through electrokinetic processes. To improve detection sensitivity, we modified various experimental parameters, such as run buffer concentration, capillary wall thickness and transducer frequency. Although acoustic wave detection was feasible, the peak width and detection limit were inadequate as compared to conventional detection methods for HPLC or CE. Nevertheless, the effects of various physical and chemical relaxation processes on acoustic wave absorption were discussed, and this has shed some light on explaining some observations, which cannot be explained by density differences alone. Accordingly, the acoustic wave method is suggested to investigate these processes, as studied in ultrasonic relaxation spectroscopy, in a flow system. 相似文献
110.
[reaction: see text] N-Sulfinyl beta-amino Weinreb amides are prepared by condensation of sulfinimines with the potassium enolate of N-methoxy-N-methylacetamide. These new chiral building blocks are useful for the asymmetric synthesis of beta-amino carbonyl compounds, as illustrated here by the concise enantioselective syntheses of sedum alkaloids (+)-sedridine and (-)-allosedridine. 相似文献