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   4710篇
  免费   398篇
  国内免费   404篇
化学   4063篇
晶体学   36篇
力学   148篇
综合类   15篇
数学   134篇
物理学   1116篇
  2024年   5篇
  2023年   105篇
  2022年   109篇
  2021年   145篇
  2020年   208篇
  2019年   175篇
  2018年   131篇
  2017年   114篇
  2016年   162篇
  2015年   170篇
  2014年   184篇
  2013年   505篇
  2012年   350篇
  2011年   245篇
  2010年   192篇
  2009年   260篇
  2008年   229篇
  2007年   289篇
  2006年   275篇
  2005年   226篇
  2004年   228篇
  2003年   199篇
  2002年   117篇
  2001年   94篇
  2000年   85篇
  1999年   81篇
  1998年   66篇
  1997年   66篇
  1996年   60篇
  1995年   69篇
  1994年   50篇
  1993年   46篇
  1992年   37篇
  1991年   37篇
  1990年   24篇
  1989年   16篇
  1988年   27篇
  1987年   24篇
  1986年   16篇
  1985年   8篇
  1984年   11篇
  1983年   4篇
  1982年   11篇
  1981年   9篇
  1980年   11篇
  1979年   7篇
  1978年   4篇
  1975年   7篇
  1974年   3篇
  1973年   4篇
排序方式: 共有5512条查询结果,搜索用时 15 毫秒
1.
大气中Pb、Cd、Cr、Mn、Ni、Tl等重金属污染物是目前国内外城市大气污染的主要因子之一,研究大气降尘中重金属元素含量具有重大意义。本文采用盐酸-硝酸混合酸为消解体系,在105℃条件下用电热板消解回流大气降尘样品2小时后定容测定,通过电感耦合等离子发射光谱法仪(ICP-AES)和电感耦合等离子体质谱仪(ICP-MS)测定大气降尘中铅、镉、铬、锌、锰、镍、铜和铊等8种重金属元素。测定结果表明: ICP-AES(铅、镉、铬、锌、锰、镍、铜)和ICP-MS(铊)两种方法的曲线线性好,准确度高,测定范围宽,检出限在0.024mg/kg-0.548mg/kg之间,精密度在0.15%-2.38%之间,能准确测定大气降尘中的重金属元素含量。  相似文献   
2.
Ultrasound-assisted soil washing processes were investigated for the removal of heavy metals (Cu, Pb, and Zn) in real contaminated soils using HCl and EDTA. The ultrasound-assisted soil washing (US/Mixing) process was compared with the conventional soil washing (Mixing) process based on the mechanical mixing. High removal efficiency (44.8% for HCl and 43.2% for EDTA) for the metals was obtained for the most extreme conditions (HCl 1.0 M or EDTA 0.1 M and L:S = 10:1) in the Mixing process. With the aide of ultrasound, higher removal efficiency (57.9% for HCl and 50.0% for EDTA) was obtained in the same extreme conditions and similar or higher removal efficiency (e.g., 54.7% for HCl 0.5 M and L:S = 10:1 and 50.5% for EDTA 0.05 M and L:S = 5:1) was achieved even in less extreme conditions (lower HCl or EDTA concentration and L:S ratio). Therefore, it was revealed that the US/Mixing was advantageous over the conventional Mixing processes in terms of metal removal efficiency, consumption of chemicals, amount of generated washing leachate, and volume/size of washing reactor. In addition, the heavy metals removal was enhanced for the smaller soil particles in the US/Mixing process. It was due to more violent movement of smaller particles in slurry phase and more violent sonophysical effects. In order to understand the mechanism of ultrasonic desorption, the desorption test was conducted using the paint-coated beads with three sizes (1, 2, and 4 mm) for the free and attached conditions. It was found that no significant desorption/removal of paint from the beads was observed without the movement of beads in the water including floatation, collision, and scrubbing. Thus, it was suggested that the simultaneous application of the ultrasound and mechanical mixing could enhance the physical movement of the particles significantly and the very high removal/desorption could be attained.  相似文献   
3.
Adsorption process was simulated in this study for removal of Hg and Ni from water using nanocomposite materials. The used nanostructured material for the adsorption study was a combined MOF and layered double hydroxide, which is considered as MOF-LDH in this work. The data were obtained from resources and different machine learning models were trained. We selected three different regression models, including elastic net, decision tree, and Gradient boosting, to make regression on the small data set with two inputs and two outputs. Inputs are Ion type (Hg or Ni) and initial ion concentration in the feed solution (C0), and outputs are equilibrium concentration (Ce) and equilibrium capacity of the adsorbent (Qe) in this dataset. After tuning their hyper-parameters, final models were implemented and assessed using different metrics. In terms of the R2-score metric, all models have more than 0.97 for Ce and more than 0.88 for Qe. The Gradient Boosting has an R2-score of 0.994 for Qe. Also, considering RMSE and MAE, Gradient Boosting shows acceptable errors and best models. Finally, the optimal values with the GB model are identical to dataset optimal: (Ion = Ni, C0 = 250, Ce = 206.0). However, for Qe, it is different and is equal to (Ion = Hg, C0 = 121.12, Ce = 606.15). The results revealed that the developed methods of simulation are of high capacity in prediction of adsorption for removal of heavy metals using nanostructure materials.  相似文献   
4.
The different coordination behavior of the flexible yet sterically demanding, hemilabile P,N ligand bis(quinoline-2-ylmethyl)phenylphosphine ( bqmpp ) towards selected CuI, AgI and AuI species is described. The resulting X-ray crystal structures reveal interesting coordination geometries. With [Cu(MeCN)4]BF4, compound 1 [Cu2(bqmpp)2](BF4)2 is obtained, wherein the copper(I) atoms display a distorted square planar and square pyramidal geometry. The steric demand and π-stacking of the ligand allow for a short Cu⋅⋅⋅Cu distance (2.588(9) Å). CuI complex 2 [Cu4Cl3(bqmpp)2]BF4 contains a rarely observed Cu4Cl3 cluster, probably enabled by dichloromethane as the chloride source. In the cluster, even shorter Cu⋅⋅⋅Cu distances (2.447(1) Å) are present. The reaction of Ag[SbF6] with the ligand leads to a dinuclear compound ( 3 ) in solution as confirmed by 31P{1H} NMR spectroscopy. During crystallization, instead of the expected phosphine complex 3 , a tris(quinoline-2-ylmethyl)bisphenyl-phosphine ( tqmbp ) compound [Ag2(tqmbp)2](SbF6)2 4 is formed by elimination of quinaldine. The Au(I) compound [Au2(bqmpp)2]PF6 ( 5 ) is prepared as expected and shows a linear arrangement of two phosphine ligands around AuI.  相似文献   
5.
The use of single-atom metals (SAM) as catalysts of energy conversion reactions is a recent topic, which has gained popularity in the last two decades. Transition metal dichalcogenides emerged as important electrocatalysts since it was discovered that their chalcogenide edge sites are active towards the electrocatalytic hydrogen evolution reaction (HER) and could also serve as supports for other metals within the same applications. Currently, several groups have reported a novel metal?chalcogenide arrangement, with the possibility of isolating metals at specific sites on chalcogenides to enhance their properties resulting in a synergistic effect in which both chalcogenide and single-atom metal features are exploited, either as promoters or active sites. Theoretical studies have been the basis of these reports.  相似文献   
6.
We report an experimental investigation of free falling super-hydrophobic (SH) spheres in glycerine-water mixtures over a wide range of Reynolds number. SH coatings have the ability to reduce the contact area between the surrounding liquid and the solid surface by entrapping an air layer in the surface roughness. We investigate the effect of this air plastron on the hydrodynamic performance of spheres, focusing our attention on the onset of wake instabilities. Our results emphasise the key role of the surface roughness properties on the triggering of wake instabilities. It is shown that, unlike what was reported in previous numerical studies on SH bluff-bodies, local deformation of the interface may act as a by-pass mechanism promoting earlier transition, yielding a decrease of the critical Reynolds number at which the wake becomes unstable. A scenario coupling the hydrodynamic instabilities (scaling with the Reynolds number) and the interface deformation (scaling with the roughness-based Weber number) is proposed to describe the different transition mechanisms in presence of SH surfaces. It is found that the promotion of wake instabilities over SH surfaces occurs when the roughness-based Weber number is larger than a critical threshold. These findings are of primary importance for guiding the design of resilient and efficient SH surfaces.  相似文献   
7.
In many perovskite oxides, ferroelectricity is intimately related to octahedral rotation patterns, which can suppress or enhance polar distortions. Using first-principles density functional theory, we investigate the relation between octahedral rotation patterns and polar instability. Based on the rotation patterns commonly observed in perovskite oxides, we present a workflow that allows to systematically and efficiently search for the unstable polar phonon modes and identify metastable polar structures. We apply the workflow to investigate rotation-pattern-dependent polar phases of CaSnO3 and find metastable polar structures by changing rotation patterns. Further investigation of the polar R3c structures shows sizable polarization comparable to the conventional ferroelectrics. We discuss substrate materials having the potential to stabilize the polar structure. Our work provides an efficient way to identify new polar phases by changing the octahedral rotation patterns, which will be useful to design new functional materials in the thin-film/substrate configuration utilizing interfacial coupling.  相似文献   
8.
9.
Cyclic organic amines are emerging as excellent building blocks to assemble organic–inorganic hybrid phase transition materials due to their flexible cyclic structure. Here, we have assembled a 1D organic-inorganic hybrid dielectric material C5H6NOPbBr3 ( 1 ) by alloying the cyclic organic amine 3-hydroxypyridine. 1 displays a remarkable switchable dielectric response induced by an order-disorder transformation of the organic moiety, this transformation behaviour is confirmed by DSC and Hirshfeld surface measurements. More interestingly, 1 has a narrowband emission (FWHM=4.64 nm) at 590 nm; FWHM is a major quality figure for narrowband photodetectors. In addition, 1 exhibits semiconducting properties with an indirect bandgap of 2.78 eV by the analysis of the UV-Vis absorption results.  相似文献   
10.
Facile synthesis of two 2-anthracene ammonium-based magnetic ionic liquids (MILs), 2-anthracene ammonium tetrachloroferrate (III) ([2A-A]FeCl4) and 2-anthracene ammonium trichlorocobaltate (II) ([2A-A]CoCl3) was performed by protonation of 2-aminoanthracene, followed complexation with FeCl3/CoCl2. The MILs were tested in the adsorptive removal of Cd2+, As3+, Pb2+ and Cr3+ from water sources. Upon treatment with 10 mg dosage of MILs in 10 mL aqueous solution of 50 ppm each of Cd2+, As3+, Pb2+ and Cr3+, adsorption capacity (mg/g) in the range of 5.73–55.5 and 23.6–56.8 for [2A-A]FeCl4 and [2A-A]CoCl3 respectively were recorded. Thus, the optimization, kinetic and isotherms studies were conducted using the [2A-A]CoCl3 adsorbent. The [2A-A]CoCl3 was more effective in pH 7–9, and equilibrium adsorption was achieved after 60 min contact time. The adsorption process proceeded via the Pseudo-second order pathway and the Langmuir isotherm model is the best fit for the adsorption process (with qmax = 227 – 357 mg/g) of all the targeted metal ions. The [2A-A]CoCl3 adsorbent demonstrated practicality with large distribution and selectivity coefficients of the targeted ions, and up to six times regeneration.  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