首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5531篇
  免费   51篇
  国内免费   1篇
化学   3060篇
晶体学   149篇
力学   188篇
数学   401篇
物理学   1785篇
  2024年   59篇
  2023年   60篇
  2022年   119篇
  2021年   129篇
  2020年   142篇
  2019年   157篇
  2018年   127篇
  2017年   169篇
  2016年   196篇
  2015年   141篇
  2014年   189篇
  2013年   398篇
  2012年   306篇
  2011年   315篇
  2010年   216篇
  2009年   195篇
  2008年   250篇
  2007年   215篇
  2006年   153篇
  2005年   150篇
  2004年   120篇
  2003年   103篇
  2002年   112篇
  2001年   67篇
  2000年   55篇
  1999年   36篇
  1998年   35篇
  1997年   39篇
  1996年   47篇
  1994年   34篇
  1993年   46篇
  1992年   54篇
  1991年   40篇
  1990年   43篇
  1989年   57篇
  1988年   47篇
  1987年   46篇
  1986年   42篇
  1985年   69篇
  1984年   64篇
  1983年   51篇
  1982年   47篇
  1981年   60篇
  1980年   57篇
  1979年   50篇
  1978年   49篇
  1977年   59篇
  1976年   60篇
  1975年   55篇
  1974年   43篇
排序方式: 共有5583条查询结果,搜索用时 0 毫秒
161.
    
Industrial waste locks are used as raw materials to reduce harmful effects on the environment and improve environmental performance. Marble clay powder can be used as a filling aid and can fill voids in concrete structures. This article will show you how to use a maximum natural sand alternative in concrete with marble powder and quarry dust. The challenge of the 21st century is to change to a new form that can support the natural system. This necessitates a radical rethinking of how to give the community infrastructure and housing. Making a concerted effort to develop novel, innovative, and alternative construction materials may be necessary. Jungles of concrete around cause's impact on the Environment and it would result in climate change. Mankind must avoid the use of things that are detrimental to the environment. So in this paper, it is decided to address the issue by adopting the use of the green concrete concept which is environmentally friendly. Green concrete is concrete made up using industrial wastes such as marble powder, quarry dust, wood ash, paper pulp, etc. Green concrete, which is capable of sustainable development, helps to reduce the consumption of natural resources, energy use, and environmental pollution. Green concrete is more cost-effective than ordinary concrete and reduces the cost of resultant concrete by 14%–20%. It is also observed that the alkali-aggregate reaction and sulfate attack resistance of concrete are both significantly improved. Green concrete is a useful tool for lowering environmental pollution and enhancing concrete's resistance to harsh conditions. All stages of infrastructure construction and rehabilitation will follow this trend of using new cement and techniques. Green concrete's adaptability and its performance derivatives will meet a variety of future needs.  相似文献   
162.
163.
The total synthesis of (+)-crassalactone B, (+)-crassalactone C, (+)-howiionol A, (+)-tricinnamate, (+)-goniofufurone, and (+)-dicinnamoyl goniofufurone is achieved by a ‘chiron approach’ starting from diacetone d-glucose (DAG). Mitsunobu inversion, Wittig olefination and ring closing metatheses were used as key steps for (+)-howiionol A and (+)-tricinnamate. Meldrum’s acid was used for the synthesis of (+)-crassalactone C, (+)-goniofufurone, and (+)-dicinnamoyl goniofufurone. Yamaguchi esterification was used for (+)-crassalactone B, while a Grignard reaction followed by concomitant deallylation was first reported in the synthesis of (+)-dicinnamoyl goniofufurone.  相似文献   
164.
165.
Traditional anodic stripping voltammetry (ASV) involves electrodeposition (reduction) of metal ions from solution over some time scale onto a working electrode followed by stripping (oxidation) of the deposited metal in a second step, where the stripping potential and quantity of charge passed provide information about the metal identity and solution concentration, respectively. ASV has recently been extended to the analysis of metal nanoparticles (NPs), which have grown popular because of their fascinating properties tunable by size, shape, and composition. There is a need for improved methods of NP analysis, and because metal NPs can be oxidized to metal ions, ASV is a logical choice. Early studies involved metal NPs as tags for the detection of biomolecules. More recently, anodic stripping has been used to directly analyze the physical, chemical, and structural properties of metal NPs. This review highlights the stripping analysis of NP assemblies on macroelectrodes, individual NPs in solution during collisions with a microelectrode, and a single NP attached to an electrode. A surprising amount of information can be learned from this very simple, low-cost technique.  相似文献   
166.
167.
The organic carbonitrile namely methyl (6-amino-5-cyano-2-methyl-4-(2-nitrophenyl)-4H-pyran)- 3-carboxylate is synthesized via one-pot multi-component reaction at room temperature using commercially available urea as inexpensive and environmentally benign organo-catalyst and its crystal structure is determined by X-ray technique. The crystals are monoclinic, sp. gr. C2/c, a = 12.3069(8) Å, b = 9.7023(7) Å, c = 24.812(2) Å, β = 94.862(6)°, Z = 8. The dihedral angle between pyran and phenyl rings is 87.8(1)°; the pyran ring is almost planar The dihedral angle between the mean planes of phenyl ring and nitro group is 44.4(2)°. The crystal structure is stabilized by N?H···N and N?H···O hydrogen bonds. In addition, C?H···π interactions are also observed in the crystal structure.  相似文献   
168.
The structure of a liquid crystal of 4-cyano-4′-n-octyloxybiphenyl (C21H25NO) is determined by X-ray diffraction analysis. The compound crystallizes in the triclinic crystal system with unit cell parameters a = 7.322(1) Å, b = 12.693(3) Å, c = 20.393(2) Å, α = 92.45(1)°, β = 99.96(1)°, γ = 99.35(2)°, and space group $P\bar 1$ . The structure is solved by the direct method and refined to R = 0.057. Two independent molecules are located in the asymmetric unit. No short intermolecular contacts are observed in the crystal packing.  相似文献   
169.
Sharma  V.  Begam  S.  Nurjamal  K.  Brahmachari  G.  Gupta  V. K. 《Crystallography Reports》2020,65(7):1187-1190
Crystallography Reports - The indole derivative [3,3':3',3''-terindolin]-2'-one bis(dimethyl sulfoxide), C24H17N3O ⋅ 2(C2H6OS) has been synthesized using green protocol in...  相似文献   
170.
Sharma  S.  Brahmachari  G.  Gupta  V. K. 《Crystallography Reports》2020,65(7):1202-1207
Crystallography Reports - The communication deals with the eco-friendly synthesis and X-ray crystal structure of an organic carbonitrile, namely...  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号