全文获取类型
收费全文 | 1473篇 |
免费 | 32篇 |
国内免费 | 4篇 |
专业分类
化学 | 1110篇 |
晶体学 | 20篇 |
力学 | 30篇 |
数学 | 99篇 |
物理学 | 250篇 |
出版年
2023年 | 9篇 |
2021年 | 9篇 |
2020年 | 13篇 |
2019年 | 18篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 26篇 |
2015年 | 29篇 |
2014年 | 26篇 |
2013年 | 63篇 |
2012年 | 74篇 |
2011年 | 81篇 |
2010年 | 42篇 |
2009年 | 55篇 |
2008年 | 91篇 |
2007年 | 85篇 |
2006年 | 89篇 |
2005年 | 83篇 |
2004年 | 69篇 |
2003年 | 66篇 |
2002年 | 48篇 |
2001年 | 23篇 |
2000年 | 32篇 |
1999年 | 28篇 |
1998年 | 21篇 |
1997年 | 20篇 |
1996年 | 22篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 14篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 30篇 |
1984年 | 25篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1981年 | 20篇 |
1980年 | 7篇 |
1979年 | 14篇 |
1978年 | 13篇 |
1977年 | 19篇 |
1976年 | 13篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1969年 | 6篇 |
排序方式: 共有1509条查询结果,搜索用时 15 毫秒
41.
Kazunori Matsui Takayuki Kaneko Yasunori Yaginuma Masaru Ryu 《Journal of Sol-Gel Science and Technology》1997,9(3):273-277
A spin probe TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy) was dissolved in a tetraethyl orthosilicate sol-gel reaction
system and measured by electron spin resonance spectroscopy at 295 K. The nitrogen hyperfine coupling constant was from 1.64–1.66
mT in the sol-gel solutions. The values were sensitive to the ethanol-to-water ratio of the solutions. The hyperfine coupling
constant in the xerogels was 1.70 mT, which was almost the same as that in water, indicating that the probe molecules were
trapped in silica pores with water adsorbed on the silica surfaces. The motion of TEMPOL in the xerogels was considerably
slower than in the sol-gel solutions. The local viscosity estimated was from 70–90 cP. The ESR spectra of TEMPOL were altered
during the sol-gel process, indicating that adsorbed water on the silicas surfaces has an important role for trapping organic
molecules in sol-gel glasses. 相似文献
42.
Hatano T Shintani Y Aga Y Shiota S Tsuchiya T Yoshida T 《Chemical & pharmaceutical bulletin》2000,48(9):1286-1292
Two new phenolic compounds, glicophenone (1) and glicoisoflavanone (2), were isolated from commercial licorice, and their structures were elucidated on the basis of spectroscopic data. Antibacterial assays of licorice phenolics for Staphylococcus aureus, including four strains of methicillin-resistant S. aureus (MRSA), and also for Escherichia coli K12 and Pseudomonas aeruginosa PAO1, were then examined. Two compounds among them, 8-(gamma,gamma-dimethylallyl)-wighteone (21) and 3'-(gamma,gamma-dimethylallyl)-kievitone (28), showed remarkable antibacterial effects [minimum inhibitory concentrations (MICs), 8 microg/ml on the MRSA strains and methicillin-sensitive S. aureus. Licochalcone A (14), gancaonin G (20), isoangustone A (24), glyasperins C (30) and D (31), glabridin, (32), licoricidin (33), glycycoumarin (34) and licocoumarone (40) showed antibacterial effects on the MRSA strains with MIC values of 16 microg/ml. Effects on the beta-lactam resistance of the MRSA strains were also examined, and licoricidin (33) noticeably decreased the resistance of the MRSA strains against oxacillin, as shown by the reduction in the MICs of oxacillin (lower than 1/128-1/1000 in the presence of 8 microg/ml of 33, and 1/8-1/32 in the presence of 4 microg/ml of 33). Mechanistic study suggested that 33 does not inhibit the formation of penicillin-binding protein 2' (PBP2'), but affects the enzymatic function of PBP2'. 相似文献
43.
Masaru Kawakami Katherine Byrne Bhavin S Khatri Tom C B McLeish D Alastair Smith 《Chemphyschem》2006,7(8):1710-1716
The viscoelastic properties of single poly(ethylene glycol) (PEG) molecules were measured by analysis of thermally and magnetically driven oscillations of an atomic force microscope (AFM) cantilever/molecule system. The molecular and monomer stiffness and friction of the PEG polymer were derived using a simple harmonic oscillator (SHO) model. Excellent agreement between the values of these two parameters obtained by the two approaches indicates the validity of the SHO model under the experimental regimes and the excellent reproducibility of the techniques. A sharp minimum in the monomeric friction is seen at around 180 pN applied force which we propose is due to a force induced change in the shape of the energy landscape describing the conformational transition of PEG from a helical to a planar state, which in turn affects the timescale of the transition and therefore modifies the measured internal friction. A knowledge of the viscoelastic response of PEG monomers is particularly important since PEG is widely used as a linker molecule for tethering groups of interest to the AFM tip in force spectroscopy experiments, and we show here that care must be exercised because of the force-dependent viscoelastic properties of these linkers. 相似文献
44.
An on-line purification method for catecholamines was studied using a flow system equipped with an alumina microcolumn. The procedure involves catecholamine adsorption, column washing and catecholamine elution steps. The system is designed not to decompose catecholamines under alkaline conditions in the adsorption step. Flow-rates and times for different solutions delivered in each step (alkaline buffer for adsorption, water for washing and an acidic solution for elution), column length and sample volume to be loaded were systematically investigated by liquid chromatography. Under optimum conditions, norepinephrine, epinephrine, dopamine and 3,4- dihydroxybenzylamine (internal standard) can be purified with recoveries of ? 90% within 11 min with manual operation. This method was efficiently applied to urine samples and the results indicate the possibility that catecholamines in biological samples are automatically purified. 相似文献
45.
46.
47.
Dr. Emi Uchida Dr. Kouji Sakaki Dr. Yumiko Nakamura Dr. Reiko Azumi Dr. Yuki Hirai Dr. Haruhisa Akiyama Dr. Masaru Yoshida Dr. Yasuo Norikane 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(51):17391-17397
Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer. 相似文献
48.
Neomacrophorins I (1), II (2), and III (3) were isolated from the culture broth of Trichoderma sp. 1212-03, which was collected at Shirakami Mountainous area in Japan. Structural analyses disclosed that these resemble known macrophorins but possess axial-hydroxy group at C3 as well as different side chains at C7′. These are diastereomeric forms of macrophorins for 5′,6′-epoxide functionality. The NMR analyses suggested their relative configurational relationship between the C1–C15 drimene and C1′–C7′ epoxyquinone moieties. ECD spectral discussions verified them particularly for C5′,C6′-epoxyquinone (1), C5′,C6′-epoxysemiquinone (2 and 3), and 2″,3″-dihydroxybutanoate moiety in 1 and 2. The configuration of C3″-stereocenter of 3 was determined by chiral GC–MS after converting into methyl (S)-3″-hydroxybutanoate by basic of 3 methanolysis. Biological assays disclosed that 1 induces hyphal branching of Cochliobolus miyabeanus as well as cytotoxicity against human colorectal cancer COLO 201. 相似文献
49.
The interrogation of fiber-Bragg-grating (FBG) sensors using a vertical-cavity surface-emitting laser (VCSEL) is discussed. A long-wavelength (1.54 μm) VCSEL was used as a wavelength-tunable source by variation in the current. Temperature stabilization was performed with a thermoelectric device. Characteristics of temperature and strain sensing were investigated. FBGs with different reflectivities were compared. For temperature sensing, the root-mean-square error in the measurement was reduced to 1/3 that without temperature stabilization. The dependence of the measurement error on the reflectivities of the FBGs was investigated. The measurement error was larger for FBGs with lower reflectivities in both temperature and strain sensing. Improvement on the sensing with low-reflectivity FBGs is discussed. 相似文献
50.
Tsuchiya H Enoto T Yamada S Yuasa T Kawaharada M Kitaguchi T Kokubun M Kato H Okano M Nakamura S Makishima K 《Physical review letters》2007,99(16):165002
A report is made on a comprehensive observation of a burstlike gamma-ray emission from thunderclouds on the Sea of Japan, during strong thunderstorms on 6 January 2007. The detected emission, lasting for approximately 40 sec, preceded cloud-to-ground lightning discharges. The burst spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung photons originating from relativistic electrons. This ground-based observation provides the first clear evidence that strong electric fields in thunderclouds can continuously accelerate electrons beyond 10 MeV prior to lightning discharges. 相似文献