全文获取类型
收费全文 | 1695篇 |
免费 | 72篇 |
国内免费 | 15篇 |
专业分类
化学 | 1064篇 |
晶体学 | 29篇 |
力学 | 63篇 |
数学 | 255篇 |
物理学 | 371篇 |
出版年
2023年 | 10篇 |
2022年 | 53篇 |
2021年 | 47篇 |
2020年 | 48篇 |
2019年 | 57篇 |
2018年 | 60篇 |
2017年 | 58篇 |
2016年 | 76篇 |
2015年 | 58篇 |
2014年 | 76篇 |
2013年 | 174篇 |
2012年 | 115篇 |
2011年 | 141篇 |
2010年 | 92篇 |
2009年 | 95篇 |
2008年 | 107篇 |
2007年 | 105篇 |
2006年 | 67篇 |
2005年 | 57篇 |
2004年 | 51篇 |
2003年 | 44篇 |
2002年 | 35篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 12篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 10篇 |
1992年 | 12篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有1782条查询结果,搜索用时 0 毫秒
991.
α-Bromo chalcones containing 2-thiene ring were prepared in good yields by the condensation of 1-(thien-3-yl)ethanone with aromatic aldehydes, followed by bromination with bromine and selective dehydrobromination with triethyl amine at room temperature. 相似文献
992.
Maram R. Al-Dweik Jalal A. Zahra Monther A. Khanfar Mustafa M. El-Abadelah Klaus-Peter Zeller Wolfgang Voelter 《Monatshefte für Chemie / Chemical Monthly》2009,140(2):221-228
Abstract A series of ethyl 2-(substituted)-9-cyclopropyl-4-fluoro-6-oxo-1H-imidazo[4,5-h]quinoline-7-carboxylates has been prepared from ethyl 7,8-diamino-1,4-dihydroquinoline-3-carboxylate via thermally induced
reactions with model alkanoic acids or via microwave-assisted cyclocondensation with some arene carboxaldehydes. Acid-catalysed
hydrolysis of the resulting ester derivatives furnished the corresponding imidazoquinoline-7-carboxylic acids. The structures
of these new acid and ester derivatives are based on microanalytical and spectral (IR, MS, and NMR) data.
Graphical abstract
相似文献
993.
A speciation procedure for As(III) and As(V) ions in environmental samples has been presented. As(V) was quantitatively recovered on aluminum hydroxide precipitate. After oxidation of As(III) by using dilute KMnO4, the developed coprecipitation was applied to determination of total arsenic. Arsenic(III) was calculated as the difference between the total arsenic content and As(V) content. The determination of arsenic levels was performed by hydride generation atomic absorption spectrometry (HG-AAS). The analytical conditions for the quantitative recoveries of As(V) including pH, amount of aluminum as carrier element and sample volume, etc. on the presented coprecipitation system were investigated. The effects of some alkaline, earth alkaline, metal ions and also some anions were also examined. Preconcentration factor was calculated as 25. The detection limits (LOD) based on three times sigma of the blank (N: 21) for As(V) was 0.012 μg L−1. The satisfactory results for the analysis of arsenic in NIST SRM 2711 Montana soil and LGC 6010 Hard drinking water certified reference materials for the validation of the method was obtained. The presented procedure was successfully applied to real samples including natural waters for arsenic speciation. 相似文献
994.
Experimental studies on heat transfer and fluid flow of water in a vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon, have been carried out in this article. The annulus has a radius ratio (outer radius to inner radius) of 1.184 and aspect ratio (length to annular gap) equal to 352. The experiments were conducted for constant heat fluxes of 1, 2.5, 5, 7.5, 10, 12.5, and 15 kW/m2. Transient behavior during the heat-up period of the system until the steady-state condition is attained and discussed. Variation in the heat transfer coefficient and Nusselt number along the annulus height represent the developing boundary layer at the entrance and fully developed flow in the remaining length. A large drop in the differential pressure is experienced when the liquid is circulated through the flow meters, which restrict the flow due to their very small passages. Flow restriction causes mass accumulation and rise of pressure at the exit of the annulus. It also causes a decrease in liquid head in the cooling leg. An increase in the heat flux leads to an increase in the heat transfer coefficient and Nusselt number. As a result of the data analysis correlations for the average Nusselt number, Reynolds number and circulation rate have been developed in terms of the heat flux. 相似文献
995.
Solitary wave solutions to the Tzitzeica type equations obtained by a new efficient approach 下载免费PDF全文
The properties of Tzitzeica equations in nonlinear optics have received a great attention of many recent studies. In this work, the so-called generalized exponential rational function method (GERFM) has been applied for finding the analytical solution of two nonlinear partial differential equations type of equations, namely Tzitzeica-Dodd-Bullough and Tzitzeica equation. The proposed method provides a wide range of closed-form travelling solutions leading to a very effective and simply-applied method by means of a symbolic computation system. The method not only provides a general form of solutions with some free parameters but also shows potential application to other types of nonlinear partial differential equations. 相似文献
996.
Muhammad I. Mustafa 《Journal of Mathematical Analysis and Applications》2018,457(1):134-152
In this paper we consider a nonlinear viscoelastic equation with minimal conditions on the relaxation function g namely , where H is an increasing and convex function near the origin and ξ is a nonincreasing function. With only these very general assumptions on the behavior of g at infinity, we establish optimal explicit and general energy decay results from which we can recover the optimal exponential and polynomial rates when and p covers the full admissible range . We get the best decay rates expected under this level of generality and our new results substantially improve several earlier related results in the literature. 相似文献
997.
In this paper, we introduce 8×8 real matrix representations of complex split quaternions. Then, the relations between real matrix representations of split and complex split quaternions are stated. Moreover, we investigate some linear split and complex split quaternionic equations with split Fibonacci and complex split Fibonacci quaternion coefficients. Finally, we also give some numerical examples as applications of real matrix representation of complex split quaternions. 相似文献
998.
Emrah?Karaman Mustafa?Soyertem ?lknur?Atasever Güven?Email author Didem?Tozkan Mahide?Kü?ük Yal??n?Kü?ük 《Positivity》2018,22(3):783-802
In this study, some new order relations on family of sets are introduced by using Minkowski difference. The relations between these orders and the ordering cone of the vector space are obtained. It is shown that depending on the corresponding cone, these order relations are partial orders on the family of nonempty bounded sets. Some relationships between these order relations and upper and lower set less order relations are investigated. Also, two scalarizing functions are introduced in order to replace set optimization problems with respect to these partial order relations with scalar optimization problems. Moreover, necessary and sufficient optimality conditions are presented. 相似文献
999.
Mustafa Serkan Soylu Nezihe alkan Alaaddin ukurovali Ibrahim Ylmaz Orhan Büyükgüngr 《Acta Crystallographica. Section C, Structural Chemistry》2005,61(12):o725-o727
The title compound, C24H26BrN3OS, crystallizes in the triclinic space group P, with two independent molecules in the asymmetric unit. The molecules adopt an E geometry about the azomethine C=N double bond. The structure is stabilized as dimers by N—H⋯N hydrogen bonding. C—H⋯π and π–π interactions are also effective in the crystal packing. 相似文献
1000.
H. Elif Kormalı Ertürün Ayça Demirel Özel Melek Nur Ayanoğlu Özlem Şahin Mustafa Yılmaz 《Ionics》2017,23(4):917-927
In this study, poly (vinyl chloride)(PVC) membrane electrodes with/without multi-walled carbon nanotubes (MWCNTs) based on a calix[4]arene derivative for perchlorate ion were described. The influence of membrane composition, pH, conditioning solution on the potentiometric response of the electrodes was investigated. Perchlorate-selective PVC membrane electrode exhibited a slope of 47.8 ± 0.6 mV/pClO4 in the range of 1.0 × 10?7–1.0 × 10?1 mol L?1at pH 4.0 while the coated Pt electrodes with MWCNT-OH, MWCNT-COOH and MWCNT displayed slopes of 46.1 ± 0.7 mV/pClO4 (5.0 × 10?6–1.0 × 10?1 mol L?1), 50.4 ± 1.9 mV/pClO4 (1.0 × 10?6–1.0 × 10?1 mol L?1) and 44.4 ± 0.3 mV/pClO4 (1.0 × 10?5–1.0 × 10?1 mol L?1), respectively. Other response characteristics of these electrodes such as response time, lifetime and detection limit were identified, and the selectivity coefficients towards various anions were calculated by separate solution method. Moreover, the perchlorate-selective electrodes described here were successfully used as an indicator electrode for the determination of perchlorate in real samples such as tap water, river water and human urine by direct calibration method. 相似文献