全文获取类型
收费全文 | 393篇 |
免费 | 7篇 |
国内免费 | 4篇 |
专业分类
化学 | 251篇 |
晶体学 | 2篇 |
力学 | 26篇 |
数学 | 65篇 |
物理学 | 60篇 |
出版年
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 1篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 4篇 |
2014年 | 6篇 |
2013年 | 15篇 |
2012年 | 30篇 |
2011年 | 25篇 |
2010年 | 23篇 |
2009年 | 24篇 |
2008年 | 22篇 |
2007年 | 37篇 |
2006年 | 21篇 |
2005年 | 9篇 |
2004年 | 23篇 |
2003年 | 17篇 |
2002年 | 5篇 |
2001年 | 8篇 |
2000年 | 9篇 |
1999年 | 14篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有404条查询结果,搜索用时 203 毫秒
381.
PEG‐400[poly(ethylene glycol‐400)] is used as reaction medium in the one‐pot synthesis of 6‐arylimidazo[2,1‐b]thiazoles by reaction with aryl ketones, hypervalent iodine(III) sulfonate and 2‐aminothiazole. Significant rate enhancements and improved yields have been observed. 相似文献
382.
An operating system is subject to random shocks that arrive according to a non-homogeneous Poisson process and cause the system
failed. System failures experience to be divided into two categories: a type-I failure (minor), rectified by a minimal repair;
or a type-II failure (catastrophic) that calls for a replacement. An age-replacement model is studied by considering both
a cumulative repair-cost limit and a system’s entire repair-cost history. Under such a policy, the system is replaced at age
T, or at the k-th type-I failure at which the accumulated repair cost exceeds the pre-determined limit, or at any type-II failure, whichever
occurs first. The object of this article is to study analytically the minimum-cost replacement policy for showing its existence,
uniqueness, and the structural properties. The proposed model provides a general framework for analyzing the maintenance policies,
and presents several numerical examples for illustration purposes. 相似文献
383.
This work investigates how bargaining power affects negotiations between manufacturers and reverse logistics providers in reverse supply chains under government intervention using a novel three-stage reverse supply chain model for two scenarios, a reverse logistics provider alliance and no reverse logistics provider alliance. Utilizing the asymmetric Nash bargaining game, this work seeks equilibrium negotiation solutions. Analytical results indicate that the reverse logistics provider alliance increases the bargaining power of reverse logistics providers when negotiating with a manufacturer for a profitable recycled-component supply contract; however, manufacturer profits are often reduced. Particularly in the case of an recycled-component vender-dominated market, a reverse logistics alliance with extreme bargaining power may cause a counter-profit effect that results in the decreases of profits for all players involved, including buyers (i.e., manufacturers) and allied recycled-component venders (i.e., reverse logistics providers). Additional managerial insights are provided for discussion. 相似文献
384.
Three new series of catenar liquid crystals 1a–c derived from heterocyclic bisoxazoles and bisthiazoles exhibiting columnar phases were reported. All compounds 1a–c exhibited hexagonal columnar phases, which were confirmed by powder XRD diffractometer. Compounds 1a have a slightly wider temperature range of columnar phases than that of compounds 1b, which might be attributed to higher dipole polarized in 1a. A Ncell and Rar value equal to 2.54–2.76 and 19.99–20.45 Å2 within a slice of 9.0 Å thick were obtained for three derivatives 1a–c (all ns=12), indicating that a single molecule was packed within columns in Colh phases. All derivatives showed good stabilities at temperature below T=408 °C on TGA. The PL spectra of all compounds 1a–c showed one intense peak at λmax=505–510 nm, and these photoluminescent emissions originated from quinoxaline moiety. 相似文献
385.
Yin-Di Su Ching-Hsiao Cheng Wu-Fu Chen Yu-Chia Chang Yu-Hsin Chen Tsong-Long Hwang Zhi-Hong Wen Wei-Hsien Wang Lee-Shing Fang Jih-Jung Chen Yang-Chang Wu Jyh-Horng Sheu Ping-Jyun Sung 《Tetrahedron letters》2014
A novel 6,12-dichlorobriarane diterpenoid, briarenolide J (1), was isolated from an octocoral identified as Briareum sp. The structure of briarane 1 was elucidated by spectroscopic methods. Briarenolide J (1) is the first metabolite of a briarane-related natural product found to possess a chlorine atom at C-12 and this compound was found to display inhibitory effects on the generation of superoxide anions and the release of elastase by human neutrophils with IC50 values of 14.98 and 9.96 μM, respectively. 相似文献
386.
Jui‐Hsin Su Chi‐Hua Hsieh Ching‐Li Lo Chiung‐Yao Huang Chang‐Feng Dai Yao‐Haur Kuo Jyh‐Horng Sheu 《中国化学会会志》2008,55(6):1286-1289
Two new sesquiterpenoids, sinularioperoxide E ( 1 ), ethyl 5‐[(2′S,5′E)‐2′, 6′‐dimethylocta‐5′,7′‐dienyl]furan‐3‐carboxylate ( 3 ), and a new C11 terpenoid‐related carboxylic acid, (3S,6E)‐3,7‐dimethyl‐nona‐6,8‐dienoic acid ( 2 ) were isolated from a Formosan soft coral Sinularia sp. The structures of 1‐3 were elucidated on the basis of extensive spectroscopic analyses and by comparison of the spectral data with those of the related metabolites. 相似文献
387.
Po-Ya Chang Chung-Kai Chang Yu-Chun Chuang Bo-Hao Chen Yen-Chung Lai Hwo-Shuenn Sheu 《中国化学会会志》2023,70(5):1009-1016
As one of the oldest species, horsetail grass (Equisetum ramosissimum Desf.) is known as a living fossil plant, dating back to the Mesozoic era. Horsetail grass is also considered one of the most important sources of bio-silica due to its ability to accumulate high amounts of silica in all parts of the plant; various minerals can also be isolated by heat treatment. Fresh and aged horsetail grass stored for 2 years under ambient conditions was investigated by synchrotron powder X-ray diffraction (PXRD). Clear crystallites were not observed in a fresh sample stored at room temperature; surprisingly, high amounts of gypsum (CaSO4·2H2O) and syngenite (K2Ca[SO4]2·H2O) were observed in the 2-day dried and 2-year aged samples, respectively. However, crystalline silica materials were not observed. In addition, in situ thermal treatment of up to 700°C was applied to investigate the crystals and phase transitions by focusing the X-ray beam onto a single stem. In situ synchrotron PXRD revealed that dehydration occurred in gypsum in the 2-day dried sample with an increase in temperature to hemihydrate (CaSO4·xH2O, 0.5 ≤ x ≤ 0.8) and anhydrite (CaSO4). On the other hand, syngenite was transformed to calciolangbeinite (K2Ca2[SO4]3) at high temperatures in 2-year aged horsetail grass. 相似文献
388.
PY Lin KS Yeh CL Su SY Sheu T Chen KL Ou MH Lin LW Lee 《Molecules (Basel, Switzerland)》2012,17(9):10846-10863
Coumarin derivatives are used as fluorescent dyes and medicines. They also have some notable physiological effects, including the acute hepatoxicity and carcinogenicity of certain aflatoxins, the anticoagulant action of dicoumarol, and the antibiotic activity of novobicin and coumerymycin A1. Because the number of drug resistant strains is increasing at present, the synthesis of new antibacterial compounds is one of the critical methods for treating infectious diseases. Therefore, a series of coumarin-substituted derivatives, namely 4-hydroxy- and 7-hydroxycoumarins, and 3-carboxycoumarins were synthesized. 4-Hydroxycoumarin derivatives 4a-c underwent rearrangement reactions. Both 4- and 7-hydroxycoumarins were treated with activated aziridines which produced series of ring-opened products 7, 8, 10, and 11. 3-Carboxy-coumarin amide dimer derivatives 14-21 were prepared by reacting aliphatic alkylamines and alkyldiamines with PyBOP and DIEA. In this study, we use a new technique called modified micro-plate antibiotic susceptibility test method (MMAST), which is more convenient, more efficient, and more accurate than previous methods and only a small amount of the sample is required for the test. Some of the compounds were produced by reactions with acid anhydrides and demonstrated the ability to inhibit Gram-positive microorganisms. The dimer derivatives displayed lower antibacterial activities. 相似文献
389.
Huei-Jyun Shih Yen-Ju Tseng Chiung-Yao Huang Zhi-Hong Wen Chang-Feng Dai Jyh-Horng Sheu 《Tetrahedron》2012,68(1):244-249
Seven new diterpenoids, namely, flexibilisolides C–G (1–5), flexibilisin C (6), and a novel 11,12-secoflexibillin (7), along with seven known compounds, 8–14, were isolated from the Dongsha Atoll soft coral Sinularia flexibilis. The structures of the new metabolites were elucidated by extensive spectroscopic analysis and comparison of the NMR data with those of known analogues. Compounds 1, 8, and 11 were shown to exhibit moderate cytotoxic activity against HeLa and B16 cancer cell lines, and compound 10 was found to exhibit more potent cytotoxic activity against SK-Hep1 and B16 cancer cell lines. Moreover, compounds 1, 2, 8, 9, and 11–14 could significantly inhibit the accumulation of the pro-inflammatory iNOS protein and 1, 8, 11, and 14 could reduce the accumulation of COX-2 protein in LPS-stimulated RAW264.7 macrophage cells. 相似文献
390.
Two series of new symmetrical 1,3,4-oxadiazoles 1a-n and 1,3,4-thiadiazoles 1b-n were prepared and their mesomorphic properties investigated by optical microscopy, differential scanning calorimetry, and powder X-ray diffractometry. Compounds 1b-n are kinetically more stable than compounds 1a-n. Compounds 1a-n exhibited monotropic nematic or smectic C phases, whereas, compounds 1b-n exhibited enantiotropic nematic or smectic A/smectic C phases. Compounds 1b-n have higher clearing temperatures and the larger temperature ranges of mesophases, which might be attributed to the better linearity and/or larger dipole, resulted from a more polarized sulfur atom than oxygen atom incorporated. The fluorescent properties of these two series of 1,3,4-thiadiazole/oxadiazole-based derivatives were also examined. The λmax peaks of the photoluminescence spectra for compounds 1a-6 and 1b-6 measured in THF occurred at ca. 385 nm and 423 nm, respectively. Both series were blue emitters. 相似文献