全文获取类型
收费全文 | 2075篇 |
免费 | 283篇 |
国内免费 | 304篇 |
专业分类
化学 | 1677篇 |
晶体学 | 28篇 |
力学 | 98篇 |
综合类 | 25篇 |
数学 | 271篇 |
物理学 | 563篇 |
出版年
2024年 | 5篇 |
2023年 | 33篇 |
2022年 | 74篇 |
2021年 | 57篇 |
2020年 | 79篇 |
2019年 | 76篇 |
2018年 | 73篇 |
2017年 | 59篇 |
2016年 | 82篇 |
2015年 | 102篇 |
2014年 | 107篇 |
2013年 | 143篇 |
2012年 | 184篇 |
2011年 | 185篇 |
2010年 | 132篇 |
2009年 | 115篇 |
2008年 | 138篇 |
2007年 | 124篇 |
2006年 | 110篇 |
2005年 | 107篇 |
2004年 | 97篇 |
2003年 | 78篇 |
2002年 | 94篇 |
2001年 | 71篇 |
2000年 | 50篇 |
1999年 | 51篇 |
1998年 | 36篇 |
1997年 | 26篇 |
1996年 | 28篇 |
1995年 | 16篇 |
1994年 | 22篇 |
1993年 | 20篇 |
1992年 | 14篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
排序方式: 共有2662条查询结果,搜索用时 0 毫秒
151.
Determination of hexavalent chromium in traditional Chinese medicines by high‐performance liquid chromatography with inductively coupled plasma mass spectrometry 下载免费PDF全文
Peng Li Li‐min Li Jing Xia Shuai Cao Xin Hu Hong‐Zhen Lian Shen Ji 《Journal of separation science》2015,38(23):4043-4047
An analytical method that combined high‐performance liquid chromatography with inductively coupled plasma mass spectrometry has been developed for the determination of hexavalent chromium in traditional Chinese medicines. Hexavalent chromium was extracted using the alkaline solution. The parameters such as the concentration of alkaline and the extraction temperature have been optimized to minimize the interconversion between trivalent chromium and hexavalent chromium. The extracted hexavalent chromium was separated on a weak anion exchange column in isocratic mode, followed by inductively coupled plasma mass spectrometry determination. To obtain a better chromatographic resolution and sensitivity, 75 mM NH4NO3 at pH 7 was selected as the mobile phase. The linearity of the proposed method was investigated in the range of 0.2–5.0 μg L?1 (r2 = 0.9999) for hexavalent chromium. The limits of detection and quantitation are 0.1 and 0.3 μg L?1, respectively. The developed method was successfully applied to the determination of hexavalent chromium in Chloriti lapis and Lumbricus with satisfactory recoveries of 95.8–112.8%. 相似文献
152.
A new-type UV light source(206 nm) was explored for the degradation of organic pollutants in wastewater for the first time.The degradation performances of triphenyltin chloride(TPTCl),dimethyl phthalate(DMP),as well as rhodamine B(RhB) were investigated.The results indicated that removal efficiency of 50 mg/L RhB,60 mg/L DMP and 120 mg/L TPTCl can reach 88.6%, 92.5%and 89.4%for 60 min,50 min and 75 min,respectively.By comparison of removal efficiency,we found 206 nm is superior to 253.7 nm UV in wastewat... 相似文献
153.
Ying Peng Huo Xu Qiu Wei Yan Shao Lin Kun An Xian Zhang Bu Lian Quan Gu 《中国化学快报》2009,20(11):1291-1295
Series of new 2-aryl-6-styryl-2,3-dihydropyridin-4(1H)-one derivatives were synthesized in acceptable to good yields by treatment of the curcuminoids with aqueous ammonia,the mechanism was proposed.By modification of the primary products,total 13 new compounds were obtained.The structures of all products were elucidated by spectroscopy analysis including HR-MS,~1H NMR and ~(13)C NMR. 相似文献
154.
Chiral quaternary ammonium salts derived from cinchonidine have been applied to catalyze the stereoselective iodolactonizations of trans-5-aryl-4-pentenoic acids leading to a mixture of two regioselectively iodolactonized products with fair to excellent yield (37-98%) and moderate enantioselectivity (exo = 42.0% ee, endo = 31.0% ee) under mild conditions. This work is the first example of asymmetric iodolactonization reaction in the presence of less than a stoichiometric amount of chiral reagent. 相似文献
155.
K.B. Helean S.V. Ushakov A. Navrotsky J. Lian J.M. Farmer 《Journal of solid state chemistry》2004,177(6):1858-1866
High-temperature oxide melt solution calorimetry and Rietveld refinements of powder X-ray diffraction data were used to investigate the structure (Fd3m; Z=8) and energetics of a series of RE2Ti2O7 (RE=Sm-Lu) compounds with the pyrochlore structure as well as La2Ti2O7 with a layered perovskite-type structure. All of the RE-titanates were found to be stable in enthalpy with respect to their oxides. In the pyrochlore series, Lu2Ti2O7 was least stable in enthalpy (ΔHf-ox at 298 K=−56.0±4.0 kJ/mol); the most stable materials were Gd-, Eu-, and Sm2Ti2O7 with ΔHf-ox at 298 K=−113.4±2.7, −106.1±4.2, −115.4±4.2 kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations, RA/RB, decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE3+ cation vs. ΔHf-ox at 298 K is non-linear and approximately parallels the increasing “resistance” to ion-beam-induced amorphization as RA/RB decreases. 相似文献
156.
Jiang Ke QIN Xian Zhang BU Yu Fang WANG Zhi Yun DU Xing Yuan WANG Zhi Shu HUANG Lin MA Lin Kun AN Lian Quan GU 《中国化学快报》2005,16(11):1451-1454
The reactions of tanshinone IIA with aromatic aldehydes have been investigated and several 2, T-(substituted arylmethylene) bis (1,6,6-trimethyl-6,7,8,9-tetrahydrophenanthro[1,2-b]- furan- 10, 11-dione) derivatives were obtained, 相似文献
157.
Xiu-Hui Lu Xin Che Jun-Feng Han Le-Yi Shi Zhen-Xia Lian 《Journal of organometallic chemistry》2010,695(4):524-529
The mechanism of the cycloaddition reaction of forming germanic bis-heterocyclic compound between singlet germylene carbene and acetone has been investigated with MP2/6-31G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by CCSD (T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the dominant reaction pathway of the cycloadditional reaction of forming germanic bis-heterocyclic compound consists of three steps: (1) the two reactants firstly form an intermediate INT4 through a barrier-free exothermic reaction of 181.4 kJ/mol; (2) INT4 further reacts with acetone (R2) to form an intermediate (INT5), which is also a barrier-free exothermic reaction of 148.9 kJ/mol; (3) INT5 then isomerizes to a germanic bis-heterocyclic product P5 via a transition state TS5 with an energy barrier of 53.3 kJ/mol. 相似文献
158.
Lian Zhang Zhongshui Li Xiaomei Huang Lingting Ye Shen Lin 《Journal of Solid State Electrochemistry》2014,18(7):2005-2012
In this paper, H3PW12O40 (PW12)-functionalized graphene nanosheets (PW12-GNs) were prepared using a green and facile method via a UV-irradiated photoreduction process, in which PW12 was directly deposited on the GNs as a reductant and also as an anionic stabilizer. The as-prepared water-dispersive PW12-GN composite is used as matrices for electrodeposition of the interesting orchidlike Pt nanoclusters in situ. The PW12-GN composite was characterized by transmission electron microscopy (TEM) and cyclic voltammetry (CV). It was shown that the electrochemical properties of PW12 were maintained in PW12-GNs. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) confirmed that Pt had been deposited on the PW12-GN composite surface. Field emission scanning electron microscopy (FE-SEM) showed that the interesting orchidlike Pt nanoparticles were uniformly immobilized on the surface of the {PW12-GN} composite film. Cyclic voltammetry and chronoamperometric curves were used to study the electrocatalytic activity of Pt/{PW12-GN} regarding methanol oxidation in 0.5 M H2SO4. It is worthy of note that the Pt/{PW12-GN} composite film-modified electrode presents a high catalytic activity (j?=?353 mA mg?1) and better tolerance of CO towards methanol electrooxidation. 相似文献
159.
Phosphine‐Catalyzed [3+2] Cycloaddition Reactions of Azomethine Imines with Electron‐Deficient Alkenes: A Facile Access to Dinitrogen‐Fused Heterocycles 下载免费PDF全文
Zhen Li Hao Yu Honglei Liu Lei Zhang Hui Jiang Bo Wang Prof. Dr. Hongchao Guo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(6):1731-1736
An efficient method for the phosphine‐catalyzed [3+2] cycloaddition reaction of azomethine imines with diphenylsulfonyl alkenes to give dinitrogen‐fused bi‐ or tricyclic heterocyclic compounds in high yields has been described. Moreover, two phenylsulfonyl groups installed on the heterocyclic products could be conveniently removed or transformed to other functional groups, making the reaction more useful. 相似文献
160.
Yuebin Lian Wenjuan Yang Chufeng Zhang Hao Sun Zhao Deng Wenjie Xu Li Song Zhongwen Ouyang Zhenxing Wang Jun Guo Yang Peng 《Angewandte Chemie (International ed. in English)》2020,59(1):286-294
Reversible oxygen conversion is important for various green energy technologies. Herein we synthesize a series of bimetallic coordination polymers by varying the Ni/Co ratio and using HITP (HITP=2,3,6,7,10,11‐hexaiminotriphenylene) as the ligand, to interrogate the role of metal centres in modulating the activity of the oxygen reduction reaction (ORR). Co3HITP2 and Ni3HITP2 are compared. Unpaired 3d electrons in Co3HITP2 result in less coplanarity but more radical character. Thus, despite of a reduced crystallinity and conductivity, the best ORR activity, comparable to 20 % Pt/C, is obtained for Co3HITP2, showing the 3d orbital configuration of the metal centre promotes ORR. Experimental and DFT studies show a transition of ORR pathway from four‐electron for Co3HITP2 to two‐electron for Ni3HITP2. Rechargeable zinc–air batteries using Co3HITP2 as the air cathode catalyst demonstrate excellent energy efficiency and stability. 相似文献