全文获取类型
收费全文 | 5001篇 |
免费 | 108篇 |
国内免费 | 13篇 |
专业分类
化学 | 2573篇 |
晶体学 | 128篇 |
力学 | 227篇 |
数学 | 667篇 |
物理学 | 1527篇 |
出版年
2022年 | 46篇 |
2021年 | 77篇 |
2020年 | 84篇 |
2019年 | 99篇 |
2018年 | 71篇 |
2017年 | 92篇 |
2016年 | 131篇 |
2015年 | 110篇 |
2014年 | 134篇 |
2013年 | 357篇 |
2012年 | 244篇 |
2011年 | 272篇 |
2010年 | 170篇 |
2009年 | 168篇 |
2008年 | 210篇 |
2007年 | 220篇 |
2006年 | 167篇 |
2005年 | 174篇 |
2004年 | 139篇 |
2003年 | 97篇 |
2002年 | 91篇 |
2001年 | 64篇 |
2000年 | 51篇 |
1999年 | 54篇 |
1998年 | 50篇 |
1997年 | 30篇 |
1996年 | 53篇 |
1995年 | 48篇 |
1994年 | 58篇 |
1993年 | 71篇 |
1992年 | 64篇 |
1991年 | 49篇 |
1990年 | 58篇 |
1989年 | 52篇 |
1988年 | 62篇 |
1987年 | 51篇 |
1985年 | 72篇 |
1984年 | 96篇 |
1983年 | 60篇 |
1982年 | 64篇 |
1981年 | 69篇 |
1980年 | 70篇 |
1979年 | 74篇 |
1978年 | 67篇 |
1977年 | 62篇 |
1976年 | 57篇 |
1975年 | 44篇 |
1974年 | 47篇 |
1973年 | 55篇 |
1972年 | 33篇 |
排序方式: 共有5122条查询结果,搜索用时 187 毫秒
981.
Rajeev Bhatt S. Ganesamoorthy Indranil Bhaumik A.K. Karnal P.K. Gupta 《Journal of Physics and Chemistry of Solids》2012,73(2):257-261
Vapour transport equilibrium (VTE) technique was used to prepare near stoichiometric LiNbO3 (NSLN) crystals. Simultaneous occurrence of reduction has been observed during the Li-enrichment that results in the weak absorption bands centred at 1.7, 2.6 and 3.7 eV in the absorption spectrum. Annealing in oxygen atmosphere resulted in decrease in the intensity of these bands. The indirect and direct band-gap energies for NSLN crystals evaluated from absorption studies are reported. The energy of the phonon involved in the indirect transition is ~85 meV (685 cm?1). Near room temperature ac-conductivity measurements reveal lower conductivity for oxygen annealed NSLN crystal in comparison to as prepared NSLN and CLN specimens. The activation energies for ac-conductivity along the z-direction for NSLN and CLN crystals in the temperature range 500–1100 K are 1.03 eV and 0.96 eV, respectively. 相似文献
982.
Chhavi Bhanot Shruti Trivedi Arti Gupta Shubha Pandey Siddharth Pandey 《The Journal of chemical thermodynamics》2012,45(1):137-144
Correlation between the dynamic viscosity (η) and the microviscosity of a hybrid green medium constituted of water and poly(ethylene glycol) (PEG) of average molar mass (200, 400, and 600) g · mol?1, respectively, is explored over the temperatures range (10 to 90) °C across the complete composition regime. The microviscosity is obtained using a fluorescence probe 1,3-bis-(1-pyrenyl)propane (BPP), which is manifested through the ratio of the monomer-to-intramolecular excimer intensities (IM/IE). Aqueous PEG mixtures are observed to behave similar to Newtonian fluids as the temperature dependence of dynamic viscosity follows Arrhenius-type behavior. Surprisingly, a simple and convenient linear dependence of ln η with wt% PEG of the mixture is established. The BPP IM/IE is observed, in general, to increase with the bulk dynamic viscosity of the mixture having >10 wt% PEG suggesting a good correlation between the bulk dynamic viscosity and BPP-reported microviscosity when the viscosity of the aqueous PEG mixture is relatively high. 相似文献
983.
Arsenic(III) sorption on nanostructured cerium incorporated manganese oxide (NCMO): a physical insight into the mechanistic pathway 总被引:1,自引:0,他引:1
Gupta K Bhattacharya S Nandi D Dhar A Maity A Mukhopadhyay A Chattopadhyay DJ Ray NR Sen P Ghosh UC 《Journal of colloid and interface science》2012,377(1):269-276
Arsenic(III) sorption was investigated with nanostructured cerium incorporated manganese oxide (NCMO). The pH between 6.0 and 8.0 was optimized for the arsenic(III) sorption. Kinetics and equilibrium data (pH=7.0±0.2, T=303±1.6 K, and I=0.01 M) of arsenic(III) sorption by NCMO described, respectively, the pseudo-second order and the Freundlich isotherm equations well. The sorption process was somewhat complicated in nature and divided into two different segments, initially very fast sorption followed by slow intraparticle diffusion process. Sorption reaction of arsenic(III) on NCMO was endothermic (ΔH°=+13.46 kJ mol(-1)) and spontaneous (ΔG°=-24.75 to -30.15 kJ mol(-1) at T=283-323 K), which took place with increasing entropy (ΔS°=+0.14 kJ mol(-1)K(-1)) at solid-liquid interface. Energy of arsenic(III) sorption estimated by analyzing the equilibrium data using the D-R isotherm model was 15.4 kJ mol(-1), indicating the ion-exchange type mechanism. Raman, FT-IR, pH effect, desorption, etc. studies indicated that arsenic(III) was oxidized to arsenic(V) during the sorption process. 相似文献
984.
The depletion of non-renewable resources and rise in global warming has caused great concern to humankind. With a view to
use renewable source of energy and to eliminate hazardous chemical compounds from air, soil, and water, photocatalysis utilizing
solar energy is becoming a rapidly expanding technology. Semiconductor nanoparticles have the ability to undergo photoinduced
electron transfer to an adsorbed particle governed by the band energy positions of the semiconductor and the redox potential
of the adsorbate. A brief overview of metal oxides and sulphides that can act as sensitizers for light-induced redox processes
due to their electronic structure is presented here. 相似文献
985.
Naveen Gautam Shikha Gupta Neha Ajmera D. C. Gautam 《Journal of heterocyclic chemistry》2012,49(3):710-715
10H‐phenothiazines are synthesized via Smiles rearrangement. These prepared phenothiazines act as a base to prepare ribofuranosides by treating them with b‐D ‐ribofuranosyl‐1‐acetate‐2,3,5‐tribenzoate. 10H‐phenothiazines on refluxing with hydrogen peroxide in glacial acetic acid gave 10H‐phenothiazine‐5,5‐dioxides. The synthesized compounds were evaluated for their antioxidative properties through in vitro studies, and they are also screened for their antimicrobial activity. The structure of the synthesized compounds has been established by elemental analysis and spectroscopic data. 相似文献
986.
Versatile trichloromethyl carbinols can be prepared in one pot from primary alcohols by treatment with Dess-Martin periodinane (DMP) in CHCl(3) followed by introduction of commercially available 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). A modification of the method was used to convert chiral primary alcohol (R)-(-)-2,2-dimethyl-1,3-dioxolane-4-methanol to the corresponding trichloromethyl carbinol with complete stereochemical fidelity, despite the reactant proceeding through a base-sensitive aldehyde intermediate. 相似文献
987.
Ambadas B. Rode Jinkwon Kim Sang-Ho Kim Gajendra Gupta In Seok Hong 《Tetrahedron letters》2012,53(20):2571-2574
A novel S,S′-diallyl carbohydrazonodithioate derivative 3 of rhodamine B hydrazone was developed as a chemodosimeter for selective detection of mercury ions based on Hg2+ promoted cyclization. The allyl groups of 3 play a key role in the binding and selection of Hg2+ ions. The probe responds selectively to Hg2+ over various other competitive cations with marked chromo- and fluorogenic changes. The formation of stable oxadiazole derivative 8 was a strong driving force for this high selectivity. Practically, this probe is more promising because of the remarkable high selectivity, faster response, low detection limit, and aqueous solubility of 3. 相似文献
988.
The nonequilibrium dynamics and aging behavior of a supercooled liquid is investigated from an analysis of the correlation of density fluctuations at two different times. The dynamic correlation functions are computed by solving numerically the equations of nonlinear fluctuating hydrodynamics. The aging time dependence follows a modified stretched exponential form with a relaxation time which is dependent on the aging time. This is similar to the behavior seen in the aging data of dielectric response functions of a typical glass forming liquid. 相似文献
989.
As part of a comprehensive review on molecular oxygen spectroscopy, we have measured rotational spectra of isotopic forms of molecular oxygen in its a(1)Δ(g) electronic state with high-resolution terahertz spectroscopy. The data are recorded in close proximity to predicted positions. Due to the high resolution and good signal-to-noise ratio, the fundamental hyperfine parameters eQq and C(I) are determinable for (17)O-substituted species for the first time. A refined nuclear spin orbit coupling constant, a = -211.9328(283) MHz, was determined, and is roughly two orders of magnitude more precise than values determined from near infrared spectroscopy or electron spin resonance studies. Vibrationally excited oxygen in the a(1)Δ(g) electronic state was also observable with small signal levels for many of the rotational transitions. 相似文献
990.
To elucidate the reactive behavior of RDX crystals at pressures and temperatures relevant to shock wave initiation, Raman spectroscopy and optical imaging were used to determine the pressure-temperature (P-T) stability and the decomposition of γ-RDX, the high pressure phase of RDX. Experiments were performed on single crystals in a diamond anvil cell at pressures from 6 to 12 GPa and at temperatures up to 600 K. Evidence for the direct decomposition of γ-RDX above 6 GPa, without the involvement of other phases, is provided. The upper limit of the P-T locus for the γ-RDX thermal decomposition was determined. A refined P-T phase diagram of RDX is presented that includes the current findings for γ-RDX. The static compression results are used to gain key insight into the shock initiation of RDX, including a determination of the RDX phase at decomposition and understanding the role of pressure and temperature in accelerating shock induced decomposition. This study has established the important role that γ-RDX plays in decomposition of RDX under static and shock compression conditions; thus theoretical modeling of RDX decomposition at high pressures and temperatures needs to incorporate the γ-phase response. 相似文献