全文获取类型
收费全文 | 1136篇 |
免费 | 57篇 |
国内免费 | 81篇 |
专业分类
化学 | 541篇 |
晶体学 | 5篇 |
力学 | 114篇 |
综合类 | 14篇 |
数学 | 295篇 |
物理学 | 305篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 8篇 |
2021年 | 17篇 |
2020年 | 15篇 |
2019年 | 16篇 |
2018年 | 15篇 |
2017年 | 35篇 |
2016年 | 18篇 |
2015年 | 19篇 |
2014年 | 35篇 |
2013年 | 68篇 |
2012年 | 41篇 |
2011年 | 48篇 |
2010年 | 44篇 |
2009年 | 52篇 |
2008年 | 63篇 |
2007年 | 71篇 |
2006年 | 59篇 |
2005年 | 54篇 |
2004年 | 47篇 |
2003年 | 43篇 |
2002年 | 154篇 |
2001年 | 43篇 |
2000年 | 37篇 |
1999年 | 27篇 |
1998年 | 25篇 |
1997年 | 21篇 |
1996年 | 22篇 |
1995年 | 17篇 |
1994年 | 19篇 |
1993年 | 23篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有1274条查询结果,搜索用时 31 毫秒
31.
IR lattice vibration spectra were used to monitor the changes in the structure of zeolite Na-Y upon its dealumination with ethylenediaminetetraacetic acid (EDTA) or with HCl. The terminal Si-O(H,Na) bonds thus formed (which are detectable by characteristic absorbance at v 900–950 cm–1) are energetically less uniform when EDTA has been used as the dealuminating agent. This inhomogeneity is connected with the local deformations of zeolite structure which result in lowering the symmetry of silicon-oxygen tetrahedra. As a consequence, two novel bands appear at v 1090 cm–1 and 1200 cm–1 in addition to the fundamental absorption bands, vas (TO4) (T = Si, Al), at 1030 cm–1 and 1145 cm–1. A mechanism of dealumination is proposed, which takes into account the topochemical peculiarities involved in the chelation of the framework aluminum ions with EDTA.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskay, No. 2, pp. 284–287, February, 1993. 相似文献
32.
33.
The interaction of a nonspherical metallic or nonmetallic particle with a rarefied thermal plasma flow is considered. Heat transfer to a particle of arbitrary shape with an extremely thin plasma sheath due to, respectively, gas molecules, electrons, and ions is described. Analytical expressions are derived for charge and heat fluxes in the particular case of a spheroidal metallic or nonmetallic particle in a subsonic plasma flow. It has been shown that the intensity of heat exchange is greatly influenced by gas ionization, charge transfer processes, and particle shape, velocity, and orientation in the plasma flow. 相似文献
34.
Summary Based on a thorough knowledge of the actual system precision significance testing of the primary eigen values, resulting from principal component analysis of the two-dimensional data array of HPLC with photodiode-array detection, is a powerful means to uncover unresolved chromatographic peaks. The implementation of this chemometric technique for assuring peak homogeneity and results showing the efficiency for two-component peaks in regard to spectral characteristics, chromatographic resolution and absorbance ratio of the investigated compounds are presented. 相似文献
35.
Robert Martin Nicole Gros Volker Böhmer Hermann Kämmerer 《Monatshefte für Chemie / Chemical Monthly》1979,110(5):1057-1066
TheFries rearrangement of different methoxy benzoates has been investigated. Frompara-methoxy benzoates the corresponding hydroxy-4-methoxy benzophenones could be obtained in good yields by treatment withLewis acids (especially TiCl4) in nitromethane at 20°C (4-hydroxy derivatives) or without solvent at 120°C (2-hydroxy derivatives). Under the same conditions only demethylation occurs withortho-methoxy benzoates leading to the corresponding salicylates. Small amounts of hydroxy-2-methoxy benzophenones were obtained by treatment with polyphosphoric acid. 相似文献
36.
负载Pt、Pd催化剂上NO-TPD和NO催化还原性能1)钟依均(浙江师范大学化学系金华321004)罗孟飞周碧袁贤鑫(杭州大学催化研究所杭州310028)关键词负载Pt、Pd催化剂NO-TPDCO-NO反应分类号O643.32氮氧化物(NOx)污染是... 相似文献
37.
固固相反应合成牛磺酸水杨醛钾与锑、铋的配合物 总被引:2,自引:0,他引:2
合成了牛磺酸水杨醛钾,并采用室温固固相反应法合成了牛磺酸水杨醛钾与三氯化锑和三氯化铋的配合物,其组成为:K2MC18H20O8N2S2 (M = Sb, Bi)。两种配合物的晶体结构均属于单斜晶系,锑配合物的晶胞参数为:a = 1.2869 nm, b = 1.7636 nm, c = 1.9917 nm, β= 93.79埃活榕浜衔锏木О问篴 = 1.4770 nm, b = 2.0334 nm, c = 2.0149 nm, β= 94.05。红外光谱表明N、Cl原子参与了配位,中心离子的配位数为5。 相似文献
38.
Biswanath Das D. Nandan Kumar Keetha Laxminarayana B. Ravikanth 《Helvetica chimica acta》2007,90(7):1330-1334
The synthesis of 14‐aryl‐ or 14‐alkyl‐14H‐dibenzo[a,j]xanthenes 3 involving the treatment of naphthalen‐2‐ol ( 1 ) with arenecarboxaldehydes or alkanals 2 in the presence of HClO4?SiO2 as a heterogeneous catalyst was achieved (Table 1), and this reaction was extended to the preparation of N‐[(2‐hydroxynaphthalen‐1‐yl)methyl]amides 5 by a three‐component reaction with urea ( 4a ) or an amide 4b – d as a third reactant (Table 2). 相似文献
39.
诱导效应指数与含硫含氮的低碳直链有机化合物的标准汽化热 总被引:2,自引:0,他引:2
本文利用诱导效应指数建立了含硫含氮的低碳直链有机物的标准汽化热的简单估算方法,我们利用此法计算了一些含硫含氮的直链有机物的标准汽化热,41个可比较值的相对误差均在5%以内。 相似文献
40.
Introduction Tremendous advances have been witnessed during the last decade in the syntheses and application of Group 4 metal complexes because of the motivity which comes from both academic and industrial field. Among them, studies have been dominated by the complexes supported by bis or mono cyclopentadienyl ligands, such as Cp2MX2 or half sandwich amide (CGC) com-plexes.1-6 At that time chemists paid a little attention to the non-cyclopentadienyl complexes, but in recent years, many chem… 相似文献