全文获取类型
收费全文 | 3537篇 |
免费 | 154篇 |
国内免费 | 33篇 |
专业分类
化学 | 2593篇 |
晶体学 | 18篇 |
力学 | 162篇 |
数学 | 313篇 |
物理学 | 638篇 |
出版年
2023年 | 32篇 |
2022年 | 151篇 |
2021年 | 132篇 |
2020年 | 133篇 |
2019年 | 133篇 |
2018年 | 131篇 |
2017年 | 102篇 |
2016年 | 168篇 |
2015年 | 125篇 |
2014年 | 176篇 |
2013年 | 357篇 |
2012年 | 231篇 |
2011年 | 242篇 |
2010年 | 197篇 |
2009年 | 153篇 |
2008年 | 173篇 |
2007年 | 133篇 |
2006年 | 125篇 |
2005年 | 73篇 |
2004年 | 88篇 |
2003年 | 81篇 |
2002年 | 57篇 |
2001年 | 31篇 |
2000年 | 35篇 |
1999年 | 29篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 20篇 |
1992年 | 23篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 21篇 |
1988年 | 24篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 22篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有3724条查询结果,搜索用时 0 毫秒
71.
Neeraj Pant N. Pradhan Mohammad Hassan Murad 《International Journal of Theoretical Physics》2014,53(11):3958-3969
We present a spherically symmetric solution of the general relativistic field equations in isotropic coordinates for perfect charged fluid, compatible with a super dense star modeling. The solution is well behaved for all the values of Schwarzschild parameter u lying in the range 0 < u < 0.1727 for the maximum value of charge parameter K = 0.08163. The maximum mass of the fluid distribution is calculated by using stellar surface density as ρ b = 4.6888×1014g cm?3. Corresponding to K = 0.08 and u max = 0.1732, the resulting well behaved solution has a maximum mass M = 0.9324M ⊙ and radius R = 8.00 and by assuming ρ b = 2×1014g cm?3 the solution results a stellar configuration with maximum mass M = 1.43M ⊙ and radius R b = 12.25 km. The maximum mass is found increasing with increasing K up to 0.08. The well behaved class of relativistic stellar models obtained in this work might has astrophysical significance in the study of internal structure of compact star such as neutron star or self-bound strange quark star like Her X-1. 相似文献
72.
Taher Niknam Mohammad Hassan Khooban Abdollah Kavousifard Mohammad Reza Soltanpour 《Nonlinear dynamics》2014,75(1-2):73-83
This study deals with the problem of controlling a class of uncertain nonlinear systems in the presence of external disturbances. To achieve this goal, a new Optimal Type-2 Fuzzy Sliding Mode Controller (OT2FSMC) is introduced. In the proposed controller, a novel heuristic algorithm, namely particle swarm optimization with random inertia weight (RNW–PSO), is employed. To achieve an optimal performance, the parameters of the proposed controller as well as the input and output membership functions are optimized simultaneously by RNW–PSO. The globally asymptotic stability of the closed-loop system is mathematically proved. Finally, this method of control is applied to the inverted pendulum system as a case study. Simulation results show the system performance is desirable. 相似文献
73.
Rapid direct and induced difference spectrophotometric methods for determination of pyrithioxin in single dosage forms (tablets and syrups) are reported. The direct methods depend upon measurement of the absorbance of pyrithioxin in different media at λmax i-e at 296 nm in 0.1 M hydrochloric acid, at 328 nm in citric acid-phosphate buffer of pH 7 and at 314 nm in 0.1 M sodium hydroxide. The mean percentage recovery of the authentic samples were 100.55±0.43, 101.21±0.58 and 100.29±0.64 respectively (P=0.05). The absorbance difference methods are based upon either measurement of the difference between the acid and the alkaline solutions i-e. Δ A (Alk-Acid) at 318 nm with an accuracy of 100.72±0.88 or the absorbance difference between the acid and neutral solutions i-e Δ A (pH 7-acid) at 328 nm with an accuracy of 100.31±0.68. 相似文献
74.
75.
In this work, we report the preparation of Al2O3/V2O5 nanocomposit using vanadium and aluminum nitrate by sol–gel method. Characterization of nanocomposit was carried out by powder X‐ray diffractometry (XRD), Fourier transform infrared (FT‐IR), scanning electron microscopy (SEM), Energy‐Dispersive X‐ray (EDX) and UV spectroscopy. Then, applicability of the synthesized nanocomposit was tested as a nanocatalyst for the synthesis of diindolyl oxindole derivatives, an important class of potentially bioactive compounds. The products were obtained in good to high yields from one‐pot three‐component condensation of isatin with indole. Also, this nanocatalyst has been reused several times, without observable loss of activity. 相似文献
76.
Safaa El‐din H. Etaiw Hassan Marie Elsayed M. Shalaby Rabie S. Farag Fatma A. Elsharqawy 《应用有机金属化学》2019,33(9)
Orange prismatic crystals of the supramolecular coordination polymer (SCP) ∞3[Cu(CN)2(Me3Sn)(Pyz)], SCP 1 , were synthesized using a self‐assembly method under ambient conditions. Nanosized 1 was obtained using the same molar ratio in water by ultrasonic irradiation. SCP 1 was characterized using single‐crystal X‐ray diffraction, elemental analysis, thermal analysis and Fourier transform infrared spectroscopy. SCP 1 and its nanosized 1 particles were also examined using powder X‐ay diffraction and scanning electron microscopy. The luminescence emission of SCP 1 was studied as well as its use as a sensor for the detection of common organic solvents and metal ions. Also, the catalytic activities of nanosized 1 towards various organic dyes were investigated under ambient conditions, UV irradiation and ultrasonic irradiation. Nanosized 1 as a heterogeneous nanoparticle catalyst exhibits high catalytic activity for the degradation of eosin‐Y and acid blue dyes. The mechanism of degradation investigated using various scavenger techniques is proposed and discussed. The catalytic oxidation process is mainly caused by ?OH radicals. 相似文献
77.
Sajjad Rashidi Farshad Farzin Mehdi Shanbedi Masoud Rahimipanah Maryam Savari 《Journal of Dispersion Science and Technology》2016,37(9):1277-1286
In order to enhance the thermal properties of turbine oil (TO), three different nanoparticles (CuO, Al2O3, and TiO2) are loaded into the TO. To measure the thermal performance of nanoparticle-based TO nanofluids at laminar flow and under constant heat flux boundary conditions, an experimental setup was applied. The obtained data clearly demonstrate the positive effect of all nanoparticles on the heat transfer rate of TO. As the most important factor, the heat transfer coefficient of the abovementioned two-phase systems is increased upon increasing both the volume concentration and the flow rate. An adaptive neuro-fuzzy inference system (ANFIS) is applied for modeling the effect of critical parameters on the heat transfer coefficient of nanoparticle-TO based nanofluids numerically. The results are compared with experimental ones for training and test data. The results suggest that the developed model is valid enough and promising for predicting the extant of the heat transfer coefficient. R2 and MSE values for all data were 0.990208751 and 108.1150734, respectively. Based on the results, it is obvious that our proposed modeling by ANFIS is efficient and valid, which can be expanded for more general states. 相似文献
78.
Mohamed Abass Hassan A. Allimony Heba Hassan 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):1799-1810
Abstract Oxidation of 3-acetyl-1-alkyl-4-hydroxyquinolin-2(1H)-ones using selenium dioxide under Riley conditions was described. The oxidation reaction produced a mixture of 2 unexpected α-keto acid and its dehydrated dimer derivatives. The oxidation reaction was studied under different reaction conditions in order to maximize the yields and optimize reaction conditions. Also, 1-alkyl-4-hydroxy-3-(2-nitroacetyl)quinolin-2(1H)-one and/or 3-acetyl-1-alkyl-4-diflouro-boryloxyquinolin-2(1H)-one derivatives were subjected to the same oxidation reaction giving rise improved reaction yields and selectivity in case of the boron-complex. Alkaline degradation of the dehydrated dimers led to formation of the 4-hydroxy-2-oxoquinoline-3-carboxylic acids while under the same conditions the α-keto acids underwent deoxalylation. [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 following free supplemental files: Additional figures]. 相似文献
79.
Samaneh Mahmoudi‐GomYek Davood Azarifar Masoumeh Ghaemi Hassan Keypour Masoumeh Mahmoudabadi 《应用有机金属化学》2019,33(6)
A novel Cu (II) Schiff‐base complex immobilized on core‐shell magnetic Fe3O4 nanoparticles (Fe3O4@SPNC) was successfully designed and synthesized. The structural features of these nanoparticles were studied and confirmed by using various techniques including FT‐IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy‐dispersive X‐ray spectroscopy (EDS), vibrating sample magnetometer (VSM), X‐Ray diffraction (XRD), wavelength dispersive X‐ray spectroscopy (WDX), and inductively coupled plasma (ICP). These newly synthesized nanoparticles have been used as efficient heterogeneous catalytic system for one‐pot multicomponent synthesis of new pyrano[2,3‐b]pyridine‐3‐carboxamide derivatives. Notably, the catalyst could be easily separated from the reaction mixture by using an external magnet and reused for several successive reaction runs with no significant loss of activity or copper leaching. The present protocol benefits from a hitherto unreported MNPs‐immobilized Cu (II) Schiff‐base complex as an efficient nanocatalyst for the synthesis of newly reported derivatives of pyrano[2,3‐b]pyridine‐3‐carboxamide from one‐pot multicomponent reactions. 相似文献
80.
Mihaela Pascu Daniela-Elena Pascu Gina Alina Trăistaru Aurelia Cristina Nechifor Andrei A. Bunaciu Hassan Y. Aboul-Enein 《Journal of the Iranian Chemical Society》2014,11(2):315-321
The present study investigates the antioxidant activities of some Romanian plants, using different spectrophotometric methods (FRAP I, FRAP II, and CUPRAC). The plants investigated are hawthorn (Crataegus oxyacantha), bilberry (Vaccinium myrtillus L.), rosehip (Rosa canina), and chokeberries (Aronia melanocarpa). Hawthorn is used to treat a wide variety of inflammatory conditions, but the primary use is generally restricted for treating hypertension, ischemic heart disease, congestive heart failure, and arrhythmia. Investigations have proved the safe and reliable use of plant and plant extracts for treatment of cardiovascular disorders. 相似文献