全文获取类型
收费全文 | 2612篇 |
免费 | 104篇 |
国内免费 | 6篇 |
专业分类
化学 | 2025篇 |
晶体学 | 26篇 |
力学 | 52篇 |
数学 | 133篇 |
物理学 | 486篇 |
出版年
2022年 | 22篇 |
2021年 | 24篇 |
2020年 | 41篇 |
2019年 | 40篇 |
2018年 | 33篇 |
2017年 | 19篇 |
2016年 | 57篇 |
2015年 | 47篇 |
2014年 | 57篇 |
2013年 | 97篇 |
2012年 | 113篇 |
2011年 | 124篇 |
2010年 | 82篇 |
2009年 | 99篇 |
2008年 | 149篇 |
2007年 | 109篇 |
2006年 | 147篇 |
2005年 | 151篇 |
2004年 | 118篇 |
2003年 | 99篇 |
2002年 | 80篇 |
2001年 | 53篇 |
2000年 | 61篇 |
1999年 | 40篇 |
1998年 | 26篇 |
1997年 | 41篇 |
1996年 | 36篇 |
1995年 | 22篇 |
1994年 | 39篇 |
1993年 | 26篇 |
1992年 | 42篇 |
1991年 | 23篇 |
1990年 | 30篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 42篇 |
1986年 | 28篇 |
1985年 | 36篇 |
1984年 | 34篇 |
1983年 | 28篇 |
1982年 | 38篇 |
1981年 | 37篇 |
1980年 | 39篇 |
1979年 | 35篇 |
1978年 | 26篇 |
1977年 | 33篇 |
1976年 | 26篇 |
1975年 | 20篇 |
1974年 | 15篇 |
1973年 | 15篇 |
排序方式: 共有2722条查询结果,搜索用时 31 毫秒
31.
32.
The nature of a very high-dimensional chaotic attractor in an infinite-dimensional phase space is examined for the purpose of studying the relationships between the physical processes occurring in the real space and the characteristics of high-dimensional attractor in the phase space. We introduce two complementary bases from which the attractor is observed, one the Lyapunov basis composed of the Lyapunov vectors and the another the Fourier basis composed of the Fourier modes. We introduce the exterior subspaces on the basis of the Lyapunov vectors and observe the chaotic motion projected onto these exteriors. It is shown that a certain statistical property of the projected motion changes markedly as the exterior subspace goes out of the attractor. The origin of such a phenomenon is attributed to more fundamental features of our attractor, which become manifest when the attractor is observed from the Lyapunov basis. A counterpart of the phenomenon can be observed also on the Fourier basis because there is a statistical one-to-one correspondence between the Lyapunov vectors and the Fourier modes. In particular, a statistical property of the high-pass filtered time series reflects clearly the difference between the interior and the exterior of the attractor. 相似文献
33.
The equilibrium constants, K
2, have been determined for the proton-transfer reactions of 1-phenacylquinolinium ion, PHQ+, with several amines {triethylamine (TEA), N,N,N′,N′-tetramethylethylenediamine (ED), N,N,N′, N′-tetramethylpropanediamine (PD), N,N,N′,N′-tetramethylbutanediamine (BD), and 1,8-bis(dimethylamino-naphthalene (DMAN)} in acetonitrile (AN), AN-tetrahydrofuran (THF)
and AN-ethanol (EtOH) mixtures. The reaction was followed spectrophotometrically using a stopped-flow technique. The K
2 value decreased for DMAN and increased for TEA with increasing vol-% of THF in AN-THF mixtures. The changes in the K
2 value for ED, PD and BD changed in the order: ED, PD and BD from a pattern similar to TEA to a pattern similar to DMAN. The
change in the K
2 value for DMAN with increasing vol-% of THF in AN-THF mixtures was explained by the effect of polarity on the stability of
P−Q+ (the deprotonated product of PHQ+). The effect of THF on the K
2 value is consistent with that of the peak wavelength of the absorption spectrum of P−Q+. The change in the K
2 value for TEA, ED, PD and BD depended on the structures of the protonated bases, one of the products for this reaction. The
effect of EtOH on the K
2 value for DMAN was examined in ternary EtOH-THF-AN mixtures that contain different amounts of EtOH and whose relative permittivities
were adjusted to that of EtOH. The K
2 value increased with increasing vol-% of EtOH because of the stabilization of P−Q+ upon the formation of the hydrogen-bonded complex with EtOH. The absorption spectrum of P−Q+ demonstrated a blue shift as the vol-% of EtOH increased. 相似文献
34.
Polychlorinated biphenyls (PCBs) and chlorobenzenes (CBzs) are two classes of dioxin precursors formed in municipal solid waste incinerators (MSWIs); they produce negative health effects similar to those of dioxins. Reducing the analytical time required for determining the concentrations of these compounds in MSWIs is important for quickly evaluating their importance and assessing associated health risks. In the present study, water is used as a safe and environmentally friendly solvent in microwave-assisted extraction (MAE) for PCB and CBz analyses. MAE is compared with traditional Soxhlet extraction (SE) to determine the extraction efficiencies. The evaluation of extraction efficiencies shows that MAE has a high extraction efficiency compared with that of SE when water content is lower than 60%. Furthermore, the extraction time and organic solvent consumption are reduced with MAE compared with SE. 相似文献
35.
Matsumoto T Kamada Y Sugimoto T Tada T Noguchi S Nakazumi H Shiro M Yoshino H Murata K 《Inorganic chemistry》2003,42(17):5192-5201
Two donor molecules newly synthesized, dimethylthio- and ethylenedithio-tetrathiafulvalenothioquinone-1,3-diselenolemethides (1 and 2), were used to prepare their charge-transfer (CT) salts with a magnetic FeBr(4)(-) counteranion. For 1, a low electrical conducting 1:1 salt (1.FeBr(4)) was obtained, in which molecules of 1 are tightly dimerized in a one-dimensional (1D) stacking column. On the other hand, 2 gave a 2:1 salt (2(2).FeBr(4)) as two different kinds of plate crystals (I and II). Both I and II possess similar stacking structures of molecules of 2 in each 1D column with a half-cut pipelike structure along the c axis. However, for I, the stacking columns are aligned in the same direction along the a and b axes, while for II they are in the same direction along the a axis, but in the reverse direction along the b axis, resulting in the difference in the relative arrangement of molecules of 2 and FeBr(4)(-) ions between the two crystals. The room-temperature electrical conductivities of the single crystals of I and II were 13.6 and 12.7 S cm(-)(1), respectively. The electrical conducting behavior in I was metallic above 170 K but changed to be semiconducting with a very small activation energy of 7.0 meV in the temperature range 4-170 K. In contrast, II showed the semiconducting behavior in the whole temperature range 77-285 K. The corresponding nonmagnetic GaBr(4)(-) salts with almost the same crystal structure as I and II showed definitively different electrical conducting properties in the metal to semiconductor transition temperature in I as well as in the magnitude of activation energy in the semiconducting region of I and II. The interaction between the d spins of FeBr(4)(-) ions was weak and antiferromagnetic in both I and II, but the magnitude of the spin interaction was unexpectedly larger compared with that in the FeBr(4)(-) salt of the corresponding sulfur derivative of 2 with closer contact between the neighboring FeBr(4)(-) ions. These electrical conducting and magnetic results suggest a significant interaction between the conducting pi electrons and the d spins of FeBr(4)(-) ions located near the columns or layers. 相似文献
36.
Murakami H Kawabuchi A Matsumoto R Ido T Nakashima N 《Journal of the American Chemical Society》2005,127(45):15891-15899
The shuttling process of alpha-CyD in three rotaxanes (1-3) containing alpha-cyclodextrin (alpha-CyD) as a ring, azobenzene as a photoactive group, viologen as an energy barrier for slipping of the ring, and 2,4-dinitrobenzene as a stopper was investigated. The trans-cis photoisomerization of 1 by UV light irradiation occurred in both DMSO and water due to the movement of alpha-CyD toward the ethylene group, while the photoisomerization of 2 occurred in DMSO, but not in water. No photoisomerization was observed for 3 in both water and DMSO. The activation parameters of 1 and 1-ref in DMSO are subject to a compensation relation between deltaS(double dagger) and deltaH(double dagger); however, in water, the deltaS(double dagger) terms are not compensated by the deltaH(double dagger) terms. Alternating irradiation of the UV and visible lights resulted in a reversible change in the induced circular dichroism (ICD) bands of trans-1 and cis-1. In contrast, after the UV light irradiation, the ICD band of trans-2 decreased without the appearance of any bands of cis-2. The NMR spectra of 2 in DMSO showed coalescence of the split signals for the methylene and for the viologen protons due to the shuttling of alpha-CyD. Both the NOE differential spectra for cis-1 in water after UV light irradiation and 2 in DMSO after heating to 120 degrees C showed the negative NOE peaks assigned to interior protons of alpha-CyD, suggesting that alpha-CyD in cis-1 exists at the one ethylene moiety, and alpha-CyDs in cis-2 and 2 heated in DMSO exist at the propylene moieties. 相似文献
37.
N Kubodera H Watanabe T Kawanishi M Matsumoto 《Chemical & pharmaceutical bulletin》1992,40(6):1494-1499
Six analogues of 1 alpha,25-dihydroxy-22-oxavitamin D3 (OCT) (2), 26,27-dimethyl OCT (5), 26,27-diethyl OCT (6), 24-norOCT (7), 24-homoOCT (8), 24-dihomoOCT (9), and 24-trihomoOCT (10) were synthesized from the 20(S)-alcohol (11) as the common starting material. In the activity inducing differentiation of human myeloid leukemia cells (HL-60) into macrophages, 26,27-dimethyl OCT (5) and 24-homoOCT (8) showed the highest activities. The binding properties of these analogues to the chick embryonic intestinal 1 alpha,25-dihydroxyvitamin D3 (1) receptor are also described. 相似文献
38.
S Yokohama T Miwa S Aibara H Fujiwara H Matsumoto K Nakayama T Iwamoto M Mori R Moroi W Tsukada 《Chemical & pharmaceutical bulletin》1992,40(9):2391-2398
A series of 6-alkyl- or 6-(cycloalkylalkyl)-[1,3,4]thiadiazolo[3,2- a]-1,2,3-triazolo[4,5-d]pyrimidin-9(3H)-ones 1b--o was synthesized from the corresponding 1,3,4-thiadiazol-5-amines 3b--o and the antiallergic activities of the products were evaluated. Among the compounds 6-(2-cyclohexylethyl)- [1,3,4]thiadiazolo[3,2-a]-1,2,3-triazolo[4,5-d]pyrimidin-9(3H)-one 1h, whose X-ray crystallographic stereostructure is shown, was found to be a promising new antiallergic agent, which has low toxicity and dual activity as a leukotriene D4 receptor antagonist and as an orally active mast cell stabilizer. 相似文献
39.
T. Nagaishi S. Ishiyama M. Matsumoto S. Yoshinaga 《Journal of Thermal Analysis and Calorimetry》1984,29(1):121-129
The reactions between ammonium sulphate and three metal oxides (Cr2O3, MnO2 and Fe2O3) were studied. It was confirmed by X-ray diffraction and chemical analysis that stable reaction intermediates were formed consecutively in the course of the reactions.These were (NH4)3M(SO4)3 and NH4M(SO4)2 for Cr2O3 and Fe2O3 and (NH4)2Mn2(SO4)3 for MnO2. The thermal decompositions of these intermediates and of the metal sulphates were carried out. The contracting-volume equation was valid for the decomposition of all the intermediates. The Arrhenius parameters were determined.
Zusammenfassung Es wurden die Reaktionen zwischen Ammoniumsulfat und drei Metalloxiden (Cr2O3, MnO2 und Fe2O3) untersucht. Durch Röntgendiffraktion und chemische Analyse wurde bestätigt, dass im Verlaufe der Reaktion stabile Zwischenprodukte gebildet werden. Es handelt sich dabei um (NH4)3M(SO4)3 und NH4M(SO4)2 im Falle von Cr2O3 und Fe2O3 und um (NH4)2Mn2(SO4)3 im Falle von MnO2. Diese Zwischenprodukte und die Metallsulfate wurden thermisch zersetzt. Die Volumenkontraktionsgleichung war für die Zersetzung all dieser Zwischenprodukte gültig. Die Arrheniusparameter wurden bestimmt.
Cr2O3, MnO2 Fe2O3. , . , (NH4)3M(SO4)3 NH4M(SO4)2, (NH4)2Mn2(SO4)3. . . .相似文献
40.
Y Akiyama N Yui T Matsumoto T Iinuma T Ishikawa T Nakajima K Machida J Nishikawa M Iio K Uno 《Radioisotopes》1988,37(3):140-147
The value of preoperative bone scans in patients with primary breast and prostate cancer was evaluated prospectively. Of 414 patients with breast cancer, clinical stage I is 14, II is 219, IIIA is 59, IIIB is 39 and IV is 14. Of 88 patients with prostate cancer, clinical stage I is 14, II is 15, III is 18 and IV is 41. 11 percent of patients with breast cancer and 54 percent of patients with prostate cancer had bone metastases. Clavicle, ribs, thoracic spine, lumbar spine and pelvis metastasized most frequently. The incidences of bone metastases were 18.4% with Scirrhous carcinoma, 15.4% with Medullary tubular carcinoma and 3.8% with Papillotubular carcinoma. The methodology and results of ROC analysis were described in our other papers. Some results of data analysis were described in this paper. 相似文献