全文获取类型
收费全文 | 1711篇 |
免费 | 37篇 |
国内免费 | 3篇 |
专业分类
化学 | 709篇 |
晶体学 | 13篇 |
力学 | 58篇 |
数学 | 139篇 |
物理学 | 617篇 |
无线电 | 215篇 |
出版年
2022年 | 22篇 |
2021年 | 19篇 |
2020年 | 27篇 |
2019年 | 21篇 |
2018年 | 25篇 |
2017年 | 36篇 |
2016年 | 42篇 |
2015年 | 25篇 |
2014年 | 46篇 |
2013年 | 102篇 |
2012年 | 85篇 |
2011年 | 95篇 |
2010年 | 66篇 |
2009年 | 65篇 |
2008年 | 86篇 |
2007年 | 74篇 |
2006年 | 51篇 |
2005年 | 68篇 |
2004年 | 51篇 |
2003年 | 46篇 |
2002年 | 49篇 |
2001年 | 29篇 |
2000年 | 32篇 |
1999年 | 34篇 |
1998年 | 19篇 |
1997年 | 22篇 |
1996年 | 22篇 |
1995年 | 17篇 |
1994年 | 25篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1991年 | 27篇 |
1990年 | 20篇 |
1989年 | 25篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 20篇 |
1984年 | 17篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1979年 | 15篇 |
1978年 | 23篇 |
1977年 | 11篇 |
1976年 | 18篇 |
1975年 | 15篇 |
1974年 | 15篇 |
1973年 | 12篇 |
排序方式: 共有1751条查询结果,搜索用时 15 毫秒
101.
The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmCA*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmCA* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmCA* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmCA*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol?1 in the SmC* phase. 相似文献
102.
Mahendra Yadav Sushil Kumar Rajesh Ranjan Sinha Sumit Kumar 《Journal of Dispersion Science and Technology》2014,35(12):1751-1763
Imidazole derivatives, namely, 1-((1-(piperazinomethyl)-1H-benzoimidazol-2-yl)methyl)-2-phenylhydrazine (PBIP), and 1-((1-(morpholinomethyl)-1H-benzoimidazol-2-yl)methyl)-2-phenylhydrazine (MBIP) were synthesized and investigated as inhibitors for mild steel corrosion in 15% HCl solution using weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. It was found that the inhibition efficiency of both the inhibitors increases with increase in concentration of inhibitors and decreases with increase in temperature. The inhibitors, PBIP and MBIP, show corrosion inhibition efficiency of 92.6% and 91.4% at 300 ppm concentration, respectively, at 303 K. Polarization studies showed that both the studied inhibitors were of mixed type in nature. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were performed for surface study of uninhibited and inhibited mild steel samples. The semi-empirical AM1 method was employed for theoretical calculations. 相似文献
103.
R. K. Dutta U. C. Bind J. B. M. Krishna A. K. Sinha G. S. Taki 《Journal of Radioanalytical and Nuclear Chemistry》2014,302(2):819-824
ZnS nanoparticles implanted with 45 keV O5+ ion beam exhibited 83.6 % degradation of methyl blue in 2 h. This idea was utilized to fabricate nanocomposite system of ZnS and PMMA where ZnS nanoparticles were immobilized in PMMA film and irradiated with 45 keV O5+ ion beam at particle fluence of 2.5 × 1015, 1 × 1016 and 4 × 1016 particles/cm2. These irradiated batches of ZnS nanoparticle immobilized in PMMA batches revealed formation of porous structure characterized by scanning electron microscopy and these batches exhibited 54 % photocatalytic degradation of methyl blue in 80 min which was higher as compared to the pristine ZnS nanoparticles. 相似文献
104.
The present study investigates the effect of hybrid fillers such as graphene nanoplatelets (GnPs) and Titanium di-oxide (TiO2) in polypropylene (PP) composites on the mechanical properties. The compatibilizing agent of Maleic anhydride grafted polypropylene (MAPP) are used in the polypropylene based composites to increase the interfacial adhesion between matrix and fillers. The experiments are designed according to L16 orthogonal array (OA) based design of experiments (DOE). The parameters selected for this study are GnPs, TiO2 and MAPP with four different levels are used.By using Orthogonal array and Taguchi based experimental design, the performance characteristics of tensile modulus, tensile strength, elongation at break and toughness can be analyzed with more objective through a small set of experiments.Taguchi based analysis are used to find out the optimal parameters to maximize the tensile properties for the GnPs and TiO2 reinforced PP hybrid composites. Further, analysis of variance (ANOVA) is investigated to identify the most significant parameters which influence the mechanical properties.From the analysis it was found that the optimal parameters of 3 ?wt% GnPs, 2 ?wt% TiO2 and 6 ?wt% MAPP for maximum tensile modulus and tensile strength. The most significant parameter for tensile modulus and tensile strength is GnPs followed by TiO2 and MAPP according to ANOVA analysis. 相似文献
105.
Suhrid Banskota Hassan Brim Yun Han Kwon Gulshan Singh Sidhartha R. Sinha Huaqing Wang Waliul I. Khan Hassan Ashktorab 《Molecules (Basel, Switzerland)》2021,26(11)
Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract with an incompletely understood pathogenesis. Long-standing colitis is associated with increased risk of colon cancer. Despite the availability of various anti-inflammatory and immunomodulatory drugs, many patients fail to respond to pharmacologic therapy and some experience drug-induced adverse events. Dietary supplements, particularly saffron (Crocus sativus), have recently gained an appreciable attention in alleviating some symptoms of digestive diseases. In our study, we investigated whether saffron may have a prophylactic effect in a murine colitis model. Saffron pre-treatment improved the gross and histopathological characteristics of the colonic mucosa in murine experimental colitis. Treatment with saffron showed a significant amelioration of colitis when compared to the vehicle-treated mice group. Saffron treatment significantly decreased secretion of serotonin and pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6, in the colon tissues by suppressing the nuclear translocation of NF-κB. The gut microbiome analysis revealed distinct clusters in the saffron-treated and untreated mice in dextran sulfate sodium (DSS)-induced colitis by visualization of the Bray–Curtis diversity by principal coordinates analysis (PCoA). Furthermore, we observed that, at the operational taxonomic unit (OTU) level, Cyanobacteria were depleted, while short-chain fatty acids (SCFAs), such as isobutyric acid, acetic acid, and propionic acid, were increased in saffron-treated mice. Our data suggest that pre-treatment with saffron inhibits DSS-induced pro-inflammatory cytokine secretion, modulates gut microbiota composition, prevents the depletion of SCFAs, and reduces the susceptibility to colitis. 相似文献
106.
107.
Vipin Amoli Rashmi Tiwari Arghya Dutta Prof. Asim Bhaumik Dr. Anil Kumar Sinha 《Chemphyschem》2014,15(1):187-194
Structurally thermostable mesoporous anatase TiO2 (m‐TiO2) nanoparticles, uniquely decorated with atomically dispersed SiO2, is reported for the first time. The inorganic Si portion of the novel organosilane template, used as a mesopores‐directing agent, is found to be incorporated in the pore walls of the titania aggregates, mainly as isolated sites. This is evident by transmission electron microscopy and high‐angle annular dark field scanning transmission electron microscopy, combined with electron dispersive X‐ray spectroscopy. This type of unique structure provides exceptional stability to this new material against thermal collapse of the mesoporous structure, which is reflected in its high surface area (the highest known for anatase titania), even after high‐temperature (550 °C) calcination. Control of crystallite size, pore diameter, and surface area is achieved by varying the molar ratios of the titanium precursor and the template during synthesis. These mesoporous materials retain their porosity and high surface area after template removal and further NaOH/HCl treatment to remove silica. We investigate their performance for dye‐sensitized solar cells (DSSCs) with bilayer TiO2 electrodes, which are prepared by applying a coating of m‐TiO2 onto a commercial titania (P25) film. The high surface area of the upper mesoporous layer in the P25–m‐TiO2 DSSC significantly increases the dye loading ability of the photoanode. The photocurrent and fill factor for the DSSC with the bilayer TiO2 electrode are greatly improved. The large increase in photocurrent current (ca. 56 %) in the P25–m‐TiO2 DSSC is believed to play a significant role in achieving a remarkable increase in the photovoltaic efficiency (60 %) of the device, compared to DSSCs with a monolayer of P25 as the electrode. 相似文献
108.
Ze-Hun Cao Yu-Hao Wang Dr. Subarna Jyoti Kalita Dr. Uwe Schneider Prof. Dr. Yi-Yong Huang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(5):1900-1906
Unprecedented phosphine-catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N-based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one-pot access to cyclopentenoyl enamines and imines, or (chiral) γ-lactams through two geminal C−C bond or two C−N bond formations, respectively. Several P-based key intermediates including a 1,4-(bis)electrophilic α,β-unsaturated ketenyl phosphonium species have been detected by 31P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram-scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively. 相似文献
109.
Quantum energy coherences represent a thermodynamic resource, which can be exploited to extract energy from a thermal reservoir and deliver that energy as work. We argue that there exists a closely analogous classical thermodynamic resource, namely, energy-shell inhomogeneities in the phase space distribution of a system’s initial state. We compare the amount of work that can be obtained from quantum coherences with the amount that can be obtained from classical inhomogeneities, and find them to be equal in the semiclassical limit. We thus conclude that coherences do not provide a unique thermodynamic advantage of quantum systems over classical systems, in situations where a well-defined semiclassical correspondence exists. 相似文献
110.
The conventional method for separation of lead from a combined lead and barium sulphate precipitate by extraction with ammonium acetate has been critically studied. The results show that quantitative separation of lead is possible only when the molar concentration ratio of barium to lead is 4.2 or above, but at ratios below 4.2 the method fails because of the formation of a solid solution of lead and barium sulphates which is maximal at initial mole-ratio 0.42. The lead in the solid solution, however, forms a strong soluble complex with EDTA and can be quantitatively separated. Based on this, a gravimetric method has been worked out for determination of lead and barium in glass. 相似文献