全文获取类型
收费全文 | 499篇 |
免费 | 11篇 |
国内免费 | 1篇 |
专业分类
化学 | 343篇 |
晶体学 | 9篇 |
力学 | 42篇 |
数学 | 29篇 |
物理学 | 88篇 |
出版年
2024年 | 5篇 |
2023年 | 7篇 |
2022年 | 16篇 |
2021年 | 17篇 |
2020年 | 29篇 |
2019年 | 27篇 |
2018年 | 27篇 |
2017年 | 23篇 |
2016年 | 17篇 |
2015年 | 19篇 |
2014年 | 18篇 |
2013年 | 48篇 |
2012年 | 39篇 |
2011年 | 38篇 |
2010年 | 15篇 |
2009年 | 26篇 |
2008年 | 15篇 |
2007年 | 9篇 |
2006年 | 5篇 |
2005年 | 4篇 |
2004年 | 9篇 |
2003年 | 5篇 |
2002年 | 7篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 9篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1971年 | 1篇 |
1969年 | 3篇 |
1966年 | 1篇 |
1960年 | 1篇 |
1959年 | 2篇 |
1948年 | 1篇 |
排序方式: 共有511条查询结果,搜索用时 0 毫秒
51.
Improved graphitic carbon nitride (g-C3N4) was synthesized through a unique one-step cost-effective technique involving a dynamic gas bubbling phenomenon using ammonium chloride (NH4Cl) as a bubbling agent. An extensive investigation was carried out to optimize the weight ratio of NH4Cl and melamine during the thermal pyrolysis process. Here, we report an improved form of g-C3N4 namely “expanded g-C3N4” with increased interlayer distance and remarkable volume expansion. The surface area of this improved version has notably increased leading to higher photocatalytic efficiency as compared with its counterpart, an synthesized without adding NH4Cl. Synthesized photocatalyst materials were further used to study the Rhodamine B photodegradation under visible light. It was observed that the expanded g-C3N4 showed a 2.4 times higher photodegradation rate than its counterpart and degraded 94% of the dye in just 30 min. 相似文献
52.
Jai Devi Suman Kumari R. Malhotra 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5):587-597
Abstract The Schiff bases derived from the condensation of 2-aminobenzothiazole derivatives and 2-hydroxy-3-methoxybenzaldehyde and their silicon(IV) complexes with the general formula R2Si(L)Cl (R = Et, Bu, Ph, L = 2-(2-hydroxy-3-methoxy) benzylideneaminobenzo-thiazole) have been synthesized. These complexes have been characterized by elemental analysis, molar conductance, and spectroscopic studies including IR and NMR (1H, 13C, and 29Si) spectroscopy. The analytical data suggest trigonal bipyramidal geometry around the silicon atom in the resulting complexes. The ligands and their organosilicon complexes have also been evaluated for in vitro antimicrobial activity against bacteria (Staphylococcus aureus, Bacillus subtilis, and Escherichia coli) and fungi (Aspergillus niger and Candida albicans). The complexes were found to be more potent as compared to the ligands. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
53.
Latha Kumari S. V. Subramanyam 《Applied Physics A: Materials Science & Processing》2009,95(2):343-349
Amorphous carbon–sulfur (a-C:S) composite films were prepared by vapor phase pyrolysis technique. The structural changes in
the a-C:S films were investigated by electron microscopy. A powder X-ray diffraction (XRD) study depicts the two-phase nature
of a sulfur-incorporated a-C system. The optical bandgap energy shows a decreasing trend with an increase in the sulfur content
and preparation temperature. This infers a sulfur incorporation and pyrolysis temperature induced reduction in structural
disorder or increase in sp
2 or π-sites. The presence of sulfur (S 2p) in the a-C:S sample is analyzed by the X-ray photoelectron spectroscopy (XPS). The sp
3/sp
2 hybridization ratio is determined by using the XPS C 1s peak fitting, and the results confirm an increase in sp
2 hybrids with sulfur addition to a-C. The electrical resistivity variation in the films depends on both the sulfur concentration
and the pyrolysis temperature. 相似文献
54.
AbstractMethyl-2-(3-oxo-3-aryl) benzoates derived from acid catalyzed air oxidative fragmentation of 2-aryl-1-tetralones were efficiently undergone intramolecular-Claisen condensation in the presence of potassium tertiary butoxide. The resulting 2-benzoyl-1-indanones formed in two-step ring contractions were further subjected to indium triflate mediated retro-Claisen condensation to get 1-indanones. 相似文献
55.
Kumari Rinki Pradip K. Dutta Andy J. Hunt James H. Clark Duncan J. Macquarrie 《Macromolecular Symposia》2009,277(1):36-42
A novel chitosan-formaldehyde porous derivative (scaffolds) was prepared by reaction of 85% deacetylated chitosan with 37% aq. formaldehyde using supercritical carbon dioxide (sc. CO2). Prior to reaction, the chitosan hydrogel was prepared in 1% aq. acetic acid (AcOH) and formaldehyde. The prepared hydrogel was subjected to solvent exchange. The identity of the Schiff base was confirmed by Fourier transform infrared spectroscopy (FTIR). The chitosan-derivative was evaluated by thermal analysis, scanning electron microscopy, and porosimetry analysis. Overall, the sc. CO2 assisted chitosan derivative opens new perspectives as biomedical material. 相似文献
56.
The unsteady laminar incompressible boundary layer flow due to a two-dimensional slot jet on a flat plate at an angle of attack
has been studied. The unsteadiness in the flow field is due to the free stream velocity distribution or wall temperature (concentration)
which varies with time. The governing partial differential equations in primitive variables have been solved numerically using
an implicit finite-difference scheme in combination with the quasilinearization technique. The effect of the variation of
the free stream velocity distribution with time is found to be more pronounced on the skin friction than on the heat or mass
transfer. The Prandtl number and the variation of the wall temperature with time strongly affect the heat transfer. Similarly,
the Schmidt number and the variation of the concentration at the wall with time strongly affect the mass transfer. Beyond
a certain critical value of the viscous dissipation parameter, the plate gets heated instead of being cooled. 相似文献
57.
Solid-supported nano and microparticles of Pd(0) (SS-Pd) were prepared and used as heterogeneous catalysts for Suzuki-Miyaura cross coupling reactions of aryl halides (chloro, bromo and iodo) and phenyl boronic acid under mild and ligand-free conditions. Scanning electron microscope (SEM) and UV-vis based studies were performed to observe the distribution of nano and microparticles of palladium over solid surface and their oxidation states. In addition, the catalyst could be reused up to seven runs without significant loss of activity and stable enough under moist conditions. 相似文献
58.
Vineet Aniya Alka Kumari Rupesh Reddy B. Satyavathi 《Journal of solution chemistry》2017,46(6):1177-1201
2-Methylpropan-2-ol, an important fine chemical, may be dehydrated during extractive distillation with glycols as entrainer. Experimental isobaric phase equilibrium studies were carried out on binary mixtures of 2-methylpropan-2-ol with ethane-1,2-diol, as an entrainer, at the local atmospheric pressure of 94.99 kPa and at sub-atmospheric pressures of 19.99, 39.99, 59.99, 78.79 kPa using a Sweitoslawski-type ebulliometer. The Wilson and NRTL activity coefficient models were used to correlate the experimental results and the binary interaction parameters were obtained using the Generalized Reduced Gradient optimization technique. UNIFAC was also used to predict the deviations in bubble temperatures. Moreover, the variation in density, refractive index values and other derived properties (excess molar volumes, partial molar volumes and deviations in molar refractivity) were explored at 303.15, 313.15, 323.15 and 333.15 K to understand the shift of equilibrium with the variation in the mixture composition for the conformational state of the molecules. The Redlich–Kister polynomial equation was used to correlate the excess molar volumes and deviations from molar refractivity. Different theoretical mixing rules (Lorentz–Lorenz, Wiener, Heller, Gladstone–Dale and Arago–Biot) are investigated and reported in terms of average percentage deviation. Furthermore, the Prigogine–Flory–Patterson theory was used to predict the trend of the dependence of excess molar volumes on composition for the present system. 相似文献
59.
The steady natural convection flow on a horizontal cone embedded in a saturated porous medium with non-uniform wall temperature/concentration
or heat/mass flux and suction/injection has been investigated. Non-similar solutions have been obtained. The nonlinear coupled
differential equations under boundary layer approximations governing the flow have been numerically solved. The Nusselt and
Sherwood numbers are found to depend on the buoyancy forces, suction/injection rates, variation of wall temperature/concentration
or heat/mass flux, Lewis number and the non-Darcy parameter. 相似文献
60.
The transient boundary layer flow and heat transfer of a viscous incompressible electrically conducting non-Newtonian power-law fluid in a stagnation region of a two-dimensional body in the presence of an applied magnetic field have been studied when the motion is induced impulsively from rest. The non-linear partial differential equations governing the flow and heat transfer have been solved by the homotopy analysis method and by an implicit finite-difference scheme. For some cases, analytical or approximate solutions have also been obtained. The special interest are the effects of the power-law index, magnetic parameter and the generalized Prandtl number on the surface shear stress and heat transfer rate. In all cases, there is a smooth transition from the transient state to steady state. The shear stress and heat transfer rate at the surface are found to be significantly influenced by the power-law index N except for large time and they show opposite behaviour for steady and unsteady flows. The magnetic field strongly affects the surface shear stress, but its effect on the surface heat transfer rate is comparatively weak except for large time. On the other hand, the generalized Prandtl number exerts strong influence on the surface heat transfer. The skin friction coefficient and the Nusselt number decrease rapidly in a small interval 0<t*<1 and reach the steady-state values for t*≥4. 相似文献