全文获取类型
收费全文 | 65417篇 |
免费 | 15019篇 |
国内免费 | 5166篇 |
专业分类
化学 | 62140篇 |
晶体学 | 701篇 |
力学 | 2350篇 |
综合类 | 181篇 |
数学 | 5255篇 |
物理学 | 14975篇 |
出版年
2024年 | 95篇 |
2023年 | 686篇 |
2022年 | 949篇 |
2021年 | 1389篇 |
2020年 | 2561篇 |
2019年 | 3795篇 |
2018年 | 2128篇 |
2017年 | 1687篇 |
2016年 | 4847篇 |
2015年 | 4901篇 |
2014年 | 5234篇 |
2013年 | 6296篇 |
2012年 | 5770篇 |
2011年 | 5104篇 |
2010年 | 4728篇 |
2009年 | 4620篇 |
2008年 | 4368篇 |
2007年 | 3528篇 |
2006年 | 3159篇 |
2005年 | 2928篇 |
2004年 | 2452篇 |
2003年 | 2183篇 |
2002年 | 2950篇 |
2001年 | 2114篇 |
2000年 | 1816篇 |
1999年 | 1014篇 |
1998年 | 597篇 |
1997年 | 564篇 |
1996年 | 526篇 |
1995年 | 451篇 |
1994年 | 367篇 |
1993年 | 298篇 |
1992年 | 295篇 |
1991年 | 244篇 |
1990年 | 219篇 |
1989年 | 159篇 |
1988年 | 100篇 |
1987年 | 85篇 |
1986年 | 113篇 |
1985年 | 83篇 |
1984年 | 45篇 |
1983年 | 50篇 |
1982年 | 34篇 |
1981年 | 26篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1976年 | 7篇 |
1971年 | 3篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 507 毫秒
991.
The plasma polymerization of 4-phenylbenzonitrile was carried out with the objective of synthesizing a novel conjugated polynitrile thin film with a better optical property. The structure, compositions and morphology of the plasma-polymerized 4-phenylbenzonitrile (PPBPCN) thin films were investigated by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). A fine, homogenous PPBPCN film with a large π-conjugated system and a high retention of the aromatic ring structure of the starting monomer in the deposited plasma films is obtained when a low discharge power of 30 W was used during film formation. For the first time, a blue emission with relatively high photoluminescence intensity for PPBPCN thin films was observed. 相似文献
992.
Three new stephaoxocane‐type alkaloids, stephalonganines A–C ( 1 – 3 ), together with the known eletefine ( 4 ), were isolated from the whole plant of Stephania longa. Their structures were fully characterized spectroscopically, and the absolute configurations of the new alkaloids were assigned by comparison of their circular‐dichroism (CD) data with those of 1,2‐dihydrostephaoxocanine ( 5 ), in combination with 2D‐NMR experiments. 相似文献
993.
Determination of tetracyclines in ovine milk by high-performance liquid chromatography with a coulometric electrode array system 总被引:1,自引:0,他引:1
A method has been developed to analyze residual tetracyclines (TCs) (oxytetracycline (OTC), tetracycline (TC), chlortetracycline (CTC), methacycline (MTC), doxycycline (DC)) in ovine milk, using high-performance liquid chromatography (HPLC) with a coulometric electrode array system. The samples were pretreated, using liquid-liquid extraction based on hexane. The chromatography was performed, using a C18 column (150 mm x 4 mm i.d. and 5 microm) with a mobile phase: sodium phosphate monobasic dihydrate (pH 2.2, 0.05 M)-acetonitrile (78:22, v/v). The flow rate of mobile phase was kept constantly at 1ml/min. The residues were monitored by an ESA electrochemical detector. Potentials of four electrodes in series were set at 400, 660, 680 and 700 mV, respectively. The first electrode was set to remove those interfering substances that may co-elute with TCs and the other three electrodes were used for quantification. The maximal potential of our detection was 700 mV. Calibration curve showed good linearity and the detection limit of TCs was 12.5, 20, 25, 10 and 25 ng/ml, respectively. Optimization of the pH of the mobile phase, the proportion of acetonitrile and the pH of the pretreatment were also performed. Recoveries of TCs from spiked samples were more than 88% and the relative standard deviations were less than 4.3%. This method was reliable, sensitive, economical and suited for routine monitoring of TC residues in ovine dairy milk. 相似文献
994.
Jun-Lin Yuan Hui Zhang Hao-Hong Chen Xin-Xin Yang Jing-Tai Zhao Mu Gu 《Journal of solid state chemistry》2007,180(12):3381-3387
The crystal structures of five new alkali rare earth diphosphates were obtained by Rietveld refinement of powder X-ray diffraction (XRD) profiles, including four alkali lutetium diphosphates ALuP2O7 (A=Na, K, Rb, Cs) and the low temperature phase of KYP2O7. The scintillation properties of Ce3+-doped AREP2O7 (A=Na, K, Rb, Cs; RE=Y, Lu) powder samples were studied under static and pulsed X-ray excitations, and featured outstanding scintillation properties with light yields 1–2 times of that of Bi4(GeO4)3 and relatively short decay time of 20–28 ns. Considering the suitable emission wavelength range, large light yield, short decay time, and non-hygroscopic nature, Ce3+-doped AREP2O7-type alkali rare earth diphosphates are potential candidates for high-counting-rate scintillation applications. 相似文献
995.
液晶聚酯与环氧嵌段共聚物的合成及表征 总被引:5,自引:0,他引:5
近年来 ,人们利用高分子液晶作为热固性环氧树脂的改性剂 ,不仅可以提高环氧树脂的韧性和强度 ,而且可以改善其热性能 ,为制备高性能的环氧树脂提供了一条新的途径[1,2 ] .目前报道所使用的液晶聚合物大多为液晶聚酯[3 ] 或液晶性聚氨酯[4] ,这些液晶聚合物与环氧树脂由于存在相容性不好的问题 ,给其实际应用带来了困难 .为了改善二者的相容性 ,本文采用溶液法合成了末端带有反应活性基团的聚酯型液晶聚合物 ,将它再与双酚A环氧预聚物反应 ,制得了高分子液晶环氧嵌段共聚物 ,其合成路线如下 :2HOCOOCH3+ HO(CH2 ) 6 OH H… 相似文献
996.
The aza‐Wittig reactions of benzophenone‐, acetophenone‐ and benzaldehyde l‐[(triphenylphosphoranyl‐idene)amino]ethylidenehydrazones (4) with phthalic anhydride, 2,3‐dimethylmaleic anhydride and 7‐oxabi‐cyclo[2,2,l]hept‐5‐ene‐2,3‐dicarboxylic anhydride ( 5a ) provide a new route to 5H,7H‐1,2,4‐triazolo[1,5‐c]‐[1,3]benzoxazepin‐7‐ones 8a‐c or 6H,8H‐1,2,4‐triazolo[1,5‐c][1,3]oxazepin‐6‐ones 8d‐h via the thermal reaction of the expected azinoimine lactones 6 . 相似文献
997.
Dienone—phenol Rearrangement of C—9 Oxygenated Decalinic Dienone and Analogs through B—Ring Cleavage
JingChunCHEN GangZHOU WeiDongLI YuLinLI 《中国化学快报》2003,14(7):689-692
Dehydrogenation of 9-hydroxy decalinic enones and analogs with DDQ resulted in a formal dienone-phenol type rearrangement via B-ring cleavage, while the corresponding dienone acetates underwent base-catalyzed formal dienone-phenol type rearrangement analogously. 相似文献
998.
Macrocyclic Schiff‐base ligand, bisacetylaceton‐ethylenediimine (BAE) and its transition metal complexes M(BAE) (M = Cu2+, Ni2+) were synthesized. The complexes having characteristics of aromatic systems and well‐defined one‐dimensional structures, reacted with p‐phthaloyl chloride, to obtain polymer complexes. The complexes were characterized by elemental analysis, inductively coupled plasma (ICP), FT‐IR, and thermal analysis and show good thermal stability. ESR spectra analysis discovered that there are free radicals in the chain of polymers, indicating that a weak magnetic spin‐exchange interaction operates between the metal ions and free radicals. It is found that, as the bridging p‐phthaloyl group is able to propagate the magnetic exchange interaction, the polymer complexes show paramagnetic properties by measurement of temperature dependence of the magnetic property, and obey Curie–Weiss law. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
999.
The paper reported the synthetic procedure and character of Copper(Ⅱ) binuclear coordination compound of 1,4-bis-(1′-phenyl-3′-methyl-5′-pyrazolone-4′)-1,4-butanedione. The non-isothermal kinetic of the thermal decomposition of the complex has been studied from the TG-DTG curves by means of the Achar et al and Coats-Redfern methods. The electrochemical characterization of the complex were studied by means of powder microelectrodes technique. 相似文献
1000.
The adsorption and dissociation of O2 on Cu low-index surfaces 总被引:1,自引:0,他引:1
The extended LEPS of O(2)-Cu single crystal plane systems is constructed by means of 5-MP (the 5-parameter Morse potential). Both the adsorption and dissociation of O(2) on Cu low-index surfaces are investigated with extended LEPS in detail. All critical characteristics of the system that we obtain, such as adsorption geometry, binding energy, eigenvalues for vibration, etc., are in good agreement with the experimental results. Our calculated results suggest there are many differences between O(2)-Cu (110) and O(2)-Pd (110) systems. On a Cu (110) surface, O(2) adsorbs in a tilted configuration and there are two lowest energy dissociation channels along the [001] and [10] directions, respectively. We speculate that the adsorption geometry of O(2) on the metal surfaces relates to the lattice constant of metal. Meanwhile, We use the concepts of the molecular dissociation limit and the surface dissociation distance to analyze again the dissociation mechanism of the O(2) on the low-index surfaces. 相似文献