全文获取类型
收费全文 | 2807篇 |
免费 | 162篇 |
国内免费 | 23篇 |
专业分类
化学 | 1798篇 |
晶体学 | 54篇 |
力学 | 115篇 |
数学 | 336篇 |
物理学 | 689篇 |
出版年
2024年 | 8篇 |
2023年 | 26篇 |
2022年 | 132篇 |
2021年 | 122篇 |
2020年 | 95篇 |
2019年 | 105篇 |
2018年 | 99篇 |
2017年 | 94篇 |
2016年 | 133篇 |
2015年 | 110篇 |
2014年 | 129篇 |
2013年 | 256篇 |
2012年 | 184篇 |
2011年 | 229篇 |
2010年 | 143篇 |
2009年 | 129篇 |
2008年 | 145篇 |
2007年 | 140篇 |
2006年 | 102篇 |
2005年 | 88篇 |
2004年 | 78篇 |
2003年 | 60篇 |
2002年 | 47篇 |
2001年 | 20篇 |
2000年 | 19篇 |
1999年 | 10篇 |
1998年 | 12篇 |
1997年 | 13篇 |
1996年 | 24篇 |
1995年 | 8篇 |
1994年 | 14篇 |
1993年 | 17篇 |
1992年 | 16篇 |
1991年 | 13篇 |
1990年 | 12篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 18篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1975年 | 4篇 |
1974年 | 7篇 |
1973年 | 5篇 |
排序方式: 共有2992条查询结果,搜索用时 15 毫秒
21.
Ammar Mustafa N. M. Mikhailova N. S. Prostakov 《Chemistry of Heterocyclic Compounds》1996,32(1):65-67
9-Phenacylidene-4-azaf luorene and 9-bromo-9-(-bromophenacyl)-4-azafluorene have been used in the synthesis of spiro compounds with fragments of substituted pyrazolines. 1-Methyl-2-oxo-5 phenacylidene-indeno[2, 3-e]-1,2-dihydropyridine and hydrazine gave 1-methyl-2-oxo-3 phenylspiro[indeno(2, 3-e)-1,2-dihydropyridine-5,5-pyrazoline-2]. 相似文献
22.
Gábor Blaskó S.Fazal Hussain Alan J. Freyer Maurice Shamma 《Tetrahedron letters》1981,22(33):3127-3130
The dark yellow indenobenzazepine alkaloids lahorine () and lahoramine () have been found in Lam. (Fumariaceae). In a transformation with biogenetic implications, treatment of the spirobenzylisoquinolines dihydrofumariline () and dihydroparfumidine () with methanesulfonyl chloride and triethylamine in dry THF, followed by iodine oxidation, provided and , respectively. 相似文献
23.
Abdurrahmanoğlu S Ozkaya AR Bulut M Bekaroğlu O 《Dalton transactions (Cambridge, England : 2003)》2004,(23):4022-4029
A new sandwich-tpype dilutetium tetraphthalocyanine 4 has been synthesized by the reaction of dimeric lutetium(III) phthalocyanine 3 with two equiv. of dilithium octakishexylthiophthalocyanine in amyl alcohol. Compound 3 was prepared from 4',5',4",5"-tetraiminoisoindoline(1,4,7,10-tetrathia-12-crown-4) 1, 4,5-bis(hexylthio)-1,2-diiminoisoindoline and lutetium acetate in amyl alcohol. Compounds 3 and 4 were characterized by elemental analysis, UV/visible, IR, (1)H NMR and ESR spectroscopy. The electrochemical and electrochromic properties of 3 and 4 have been examined by cyclic voltammetry. Both 3 and 4 displayed well-defined electrochromic behavior. 相似文献
24.
Study of various binding materials like potassium bromide, poly(vinyl alcohol), starch, silver and aluminum has been carried out using laser-induced breakdown spectroscopy (LIBS). The role of matrix effects using these five binders on LIBS signal intensity was investigated for better performance of LIBS technique as a quantitative analytical tool. For comparative study of different binders, the signal intensity of different Mg lines at 518.3, 517.2, 383.8 and 279.5 nm wavelengths were recorded for pellets prepared with known concentrations of Mg in these binders. The influence of laser energy on ablated mass under different binding materials and its correlation with LIBS signal intensity has been explored. Optical scanning microscopy images of the ablated crater were studied to understand the laser ablation process. The study revealed that the binding material plays an important role in the generation of LIBS signal. The relative signal intensity measured for a standard Mg line (at 518.3 nm) were 735, 538, 387, 227 and 130 for potassium bromide, starch, poly(vinyl alcohol), silver and aluminum as binders, respectively. This indicates clearly that potassium bromide is better as a binder for LIBS studies of powder samples. 相似文献
25.
Bassam Abu Thaher Jalal A. Zahra Mustafa M. El-Abadelah Hans-Hartwig Otto 《Monatshefte für Chemie / Chemical Monthly》2004,40(9):435-439
Nitrilimines were prepared from N-arylhydrazono chlorides and reacted with -alanine yielding the corresponding amidrazones, which were treated with 1,1-carbonyldiimidazole in THF affording the hitherto unknown 3-acetyl-1-aryl-1,4,5,6-tetrahydro-1,2,4-triazepin-7-ones. 相似文献
26.
Mustafa Odabaolu idem Albayrak Reit
zkanca Fatma Zehra Aykan Peter Lonecke 《Journal of Molecular Structure》2007,840(1-3):71-89
Some new substituted polyhydroxy azo–azomethine compounds were prepared by reaction of tris(hydroxymethyl)aminomethane with (E)-2-hydroxy-5-(phenyldiazenyl) benzaldehyde and its substituted derivatives. The structures of azo and azo–azomethine compounds were determined by IR, UV–vis, 1H NMR and 13C NMR spectroscopic techniques, and/or X-ray diffraction studies. According to IR spectra, all azo–azomethine compounds adopt keto form in solid state. UV–vis analysis has shown the presence of keto–enol tautomerism in solution for all azo–azomethine compounds, except that for nitro substituted derivative, enol form is dominantly favored in solution. At the same time, above mentioned derivative compounds were studied in vitro for their antimicrobial properties. Among the phenylazosalicylaldehyde series compound tested, 4-phenylazosalicylaldehyde, 4-(3-chlorophenylazo)salicylaldehyde, 4-(2-chlorophenylazo)salicylaldehyde, 4-(4-fluorophenylazo)salicylaldehyde, 4-(3-chlorophenylazo)salicylaldehyde and 4-(4-ethylphenylazo)salicylaldehyde showed a weak antimicrobial activity only against gram positive bacteria. On the contrary, phenylazosalicylaldehyde series compounds were reacted tris(hydroxmethyl)aminomethane, that exhibited a strong antimicrobial activity against gram positive bacteria, yeast and mould. Moreover, while the 2-{[1,3-dihydroxy-2-(hydroxymethyl)propan-2-ylimino]methyl}phenol did not show an inhibition on tested microorganism, the addition of phenyldiazine groups to 2-{[1,3-dihydroxy-2-(hydroxymethyl)propan-2-ylimino]methyl}phenol resulted in a strong increases in antimicrobial activity. 相似文献
27.
Asymmetric alkylation of diarylmethane derivatives. Improved results using methoxyethoxy substituent
Alkylation of 2-methoxyethoxyphenyl phenyl methane using sec-BuLi and (−)-sparteine has been carried out in excellent yields and up to 94% ee. The best results were obtained in allylation reactions but methylation, ethylation, benzylation and trimethylsilylation have all been carried out with acceptable ee. 相似文献
28.
E. Abdel-Latif H. M. Mustafa H. A. Etman A. A. Fadda 《Russian Journal of Organic Chemistry》2007,43(3):443-448
The reaction of malononitrile dimer with phenyl isothiocyanate gave (6-amino-1-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidin-4-ylidene)malononitrile
which was then used as starting material in the synthesis of pharmacologically important fused pyrimidine derivatives, such
as 4-dicyanomethylidene-1,5-diphenyl-2-thioxo-1,2,3,4-tetrahydropyrrolo[2,3-d]pyrimidine, 6-cyano-4-dicyanomethylidene-7-methylsulfanyl-5-oxo-1-phenyl-2-thioxo-1,2,3,4,5,8-hexahydropyrido[2,3-d]pyrimidine, 6-dicyanomethylidene-3-phenyl-2-thioxo-1,2,3,6-tetrahydro-9H-purine, and 6-substituted 4-dicyanomethylidene-7-oxo-1-phenyl-2-thioxo-1,2,3,4,7,8-hexahydropteridines.
Published in Russian in Zhurnal Organicheskoi Khimii, 2007, Vol. 43, No. 3, pp. 443–449.
The text was submitted by the authors in English. 相似文献
29.
M. Aneesuddin P. N. Char M. Raza Hussain E. R. Saxena 《Journal of Thermal Analysis and Calorimetry》1983,26(2):205-215
When chalcopyrite is heated in air, up to 350? there is no marked change. Between 350 and 440?, surface material is oxidised to iron sulphate, CuSO4 and Fe2O3, while in regions not accessible to oxygen the formation of Cu5FeS4, FeS and S takes place. From 440 to 500? oxidation and sulphation phenomena occur. Stable compounds between 500 and 650? are iron sulphate, CuSO4 and Fe4O3, with a minor amount of 6CuO.Cu2O indicated at 650?. After the decomposition of iron sulphate, CuSO4 decomposes, first to CuO.CuSO4 and then to CuO. By 750? the sulphur has been totally lost from all compounds, while the oxides of copper and iron partly react to form CuFe2O4. Final products of oxidation between 800 and 850? are CuO, CuFe2O4 and Fe3O4. 相似文献
30.
In the work described here, a biosensor was developed for the determination of sulfite in food. Malva vulgaris tissue homogenate containing sulfite oxidase enzyme was used as the biological material. M. vulgaris tissue homogenate was crosslinked with gelatin using glutaraldehyde and fixed on a pretreated Teflon membrane. Sulfite was enzymatically converted to sulfate in the presence of the dissolved oxygen, which was monitored amperometrically. Sulfite determination was carried out by standard curves, which were obtained by the measurement of consumed oxygen level related to sulfite concentration. Several operational parameters had been investigated: the amounts of plant tissue homogenate and gelatin, percentage of glutaraldehyde, optimum pH and temperature. Also, some characterization studies were done. There was linearity in the range between 0.2 and 1.8 mM at 35 °C and pH 7.5. The results of real sample analysis obtained with the biosensor agreed well with the enzymatic reference method using spectrophotometric detection. 相似文献