全文获取类型
收费全文 | 18478篇 |
免费 | 1728篇 |
国内免费 | 2356篇 |
专业分类
化学 | 12162篇 |
晶体学 | 120篇 |
力学 | 985篇 |
综合类 | 115篇 |
数学 | 1004篇 |
物理学 | 2807篇 |
综合类 | 5369篇 |
出版年
2024年 | 84篇 |
2023年 | 229篇 |
2022年 | 578篇 |
2021年 | 633篇 |
2020年 | 866篇 |
2019年 | 689篇 |
2018年 | 493篇 |
2017年 | 565篇 |
2016年 | 633篇 |
2015年 | 662篇 |
2014年 | 842篇 |
2013年 | 1237篇 |
2012年 | 1072篇 |
2011年 | 940篇 |
2010年 | 735篇 |
2009年 | 893篇 |
2008年 | 914篇 |
2007年 | 1058篇 |
2006年 | 980篇 |
2005年 | 936篇 |
2004年 | 916篇 |
2003年 | 783篇 |
2002年 | 800篇 |
2001年 | 560篇 |
2000年 | 583篇 |
1999年 | 476篇 |
1998年 | 406篇 |
1997年 | 425篇 |
1996年 | 356篇 |
1995年 | 392篇 |
1994年 | 332篇 |
1993年 | 267篇 |
1992年 | 259篇 |
1991年 | 184篇 |
1990年 | 160篇 |
1989年 | 148篇 |
1988年 | 125篇 |
1987年 | 97篇 |
1986年 | 46篇 |
1985年 | 39篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 18篇 |
1980年 | 21篇 |
1979年 | 15篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
931.
A new calixcrown‐6 oligomer 5 was synthesized by reacting a 1,3‐distal di‐amino derivative of cone p‐tert‐butylcalix[4]crown‐6 with malonyl dichloride. Extraction studies with precursor monomers 2 and 3 and oligomer 5 showed differences in their metal cation extraction behavior and selectivity. Precursors monomers 2 and 3 were selective for Hg2+; whereas, oligomer 5 showed high affinity towards both alkali (Li+, Na+, K+, and Cs+) and heavy metal cations (Cu2+, Cd2+, Hg2+ and Pb2+). 相似文献
932.
A pH-responsive methylcellulose-g-poly(sodium acrylate)/attapulgite (MC-g-PNaA/APT) nanocomposite superabsorbent was prepared by the free-radical solution polymerization of methylcellulose (MC), sodium acrylate (NaA) and nanoscale attapulgite (APT) in the presence of the crosslinker N,N′-methylene-bis-acrylamide (MBA). The structure and morphology of the nanocomposite were characterized by FTIR, FESEM, TEM, XRD and EDS techniques, and the effects of the amount of MBA, MC and APT nanorods on swelling behaviors were also evaluated. Results indicate that NaA has been grafted onto MC macromolecular chains and APT nanorods participated in polymerization by its active silanol groups, and APT led to a better dispersion in the MC-g-PNaA matrix. The incorporation of APT clearly enhanced the swelling capacity and rate of the superabsorbent. In addition, the nanocomposite exhibited excellent absorption capacity on heavy metal ions, and its absorption amounts on Ni2+, Cu2+ and Zn2+ ions reached 9.86, 7.66 and 21.86 times greater than active carbon (AC). The biopolymer-based nanocomposite superabsorbents can be used as a potential water-saving material and candidate of AC for heavy metal ion absorption. 相似文献
933.
Polyamine polyethyleneimine (PEI) was first grafted on the surfaces of micro-sized silica gel particles in the manner of the coupling graft (the manner of “grafting to”), forming the grafting particles PEI/SiO2. Subsequently, via a polymer reaction, the nucleophilic substitution reaction between the primary and secondary amino groups of the grafted PEI macromolecule and chloroactic acid (CAA), iminoacetic acid groups were bonded onto the grafted PEI chains, and an iminoacetic acid (IAA)-type composite chelating material, IAA-PEI/SiO2, was formed. In this work, the preparation process of IAA-PEI/SiO2 particles was mainly researched, and the effects of the main factors on the polymer reaction, i.e., the nucleophilic substitution reaction, were examined emphatically. The adsorption behavior of IAA-PEI/SiO2 particles towards several kinds of heavy metal ions was preliminarily evaluated. The experiments results show that it is feasible to introduce IAA groups onto PEI/SiO2 particles via the substitution reaction between CAA and the amino groups of the grafted PEI. The reaction rate is affected greatly by the feed ratio of the amino group of PEI to CAA, so the substitution reaction between CAA and the amino groups of the grafted PEI is a bimolecular nucleophilic substitution reaction (SN2). The reaction temperature and the used amount of acid-acceptor NaHCO3 affect the bonding rate of IAA groups greatly. The fitting temperature was 60°C, and 1:1 of the molar ratio of NaHCO3 to CAA was an appropriate amount of acid-acceptor NaHCO3. Under the above optimal reaction conditions and with 3:1 molar ratio of amino group of PEI to CAA, 72% of the IAA group bonding rate (it is based on the hydrogen atoms in the primary and secondary amino groups of the grafted PEI) in 8 h can be reached. The composite chelating material IAA-PEI/SiO2 possesses a strong chelating adsorption ability for heavy metal ions because of the increase of the ligands and formation of stable five-membered chelate rings. 相似文献
934.
In this paper, new thermally stable isomeric unsubstituted polyesteramides have been successfully prepared by condensation of aromatic acids chlorides namely; isophthaloyl, pyridine-3,5-dicarbonyl and pyridine-2,6-pyridine-dicarbonyl dichlorides with the aminophenol isomers in NMP. Conducting the reaction in NMP/H2O (90/10 v/v) followed by centrifugal separation furnished the desired polymers as rod-like nanoparticles. The morphology of obtained nanoparticles were studied by SEM. Mixing NMP with H2O was essential for controlling the particles morphology and as a reaction accelerator. Pyridine-containing polymers exhibit semi-conducting nature as their conductivities increase with increasing temperature, while no variation of the conductivity with the temperature was observed for their corresponding phenylene analogues. Introduction of the nitro group into the polymer backbone led to a red shift in the absorption and the obtained polymers have a bright yellow color, which is unusual with this polymer group. Copper (II) ions were complexed the polyesteramides-containing nitro group in a (1:1) ratio. Complexes of pyridine-containing polymers exhibit semiconducting nature changed to metallic characters on heating and their conductivities increased tens of magnitudes than their corresponding ligands. These new types of polymeric materials and their nano-sized rods may have numerous applications in nanotechnology and their properties can be tuned for specific applications such as conducting adhesives and coating materials. 相似文献
935.
This article presents an original work aimed at rationally designing molecularly imprinted polymer (MIP) toward a high specific adsorbent. Assembling with cobalt as the pivot, the MIP was prepared by coordinating polymerizable monomers around an inducible template. The use of pivot obviously plays a positive role on increasing the specificity of MIP, so as to adsorb more for the template and less for its analogue. Related studies indicate that these may be a result of increasing specific interaction, which makes the MIP capable of recognizing the imprint species. Further information from thermodynamic analysis reveals that the increasing specific interaction, in logic, can be due to a higher fidelity of imprint, which specifically allures the template to bind. 相似文献
936.
The aim of this study is to prepare magnetic beads which can be used for the removal of heavy metal ions from synthetic solutions. Magnetic poly(ethylene glycol dimethacrylate‐vinyl imidazole) [m‐poly(EGDMA‐VIM)] beads were produced by suspension polymerization in the presence of magnetite Fe3O4 nano‐powder. The specific surface area of the m‐poly(EGDMA‐VIM) beads was found to be 63.1 m2/g with a size range of 150–200 µm in diameter and the swelling ratio was 85%. The average Fe3O4 content of the resulting m‐poly(EGDMA‐VIM) beads was 12.4%. The maximum binding capacities of the m‐poly(EGDMA‐VIM) beads were 32.4 mg/g for Cu2+, 45.8 mg/g for Zn2+, 84.2 mg/g for Cd2+and 134.5 mg/g for Pb2+. The affinity order on mass basis is Pb2+>Cd2+>Zn2+>Cu2+. Equilibrium data agreed well with the Langmuir model. pH significantly affected the binding capacity of the magnetic beads. Binding of heavy metal ions from synthetic wastewater was also studied. The binding capacities were 26.2 mg/g for Cu2+, 33.7 mg/g for Zn2+, 54.7 mg/g for Cd2+ and 108.4 mg/g for Pb2+. The magnetic beads could be regenerated up to about 97% by treating with 0.1 M HNO3. These features make m‐poly(EGDMA‐VIM) beads a potential candidate for support of heavy metal removal under magnetic field. 相似文献
937.
The article describes the syntheses and extraction properties of two p‐tert‐butylcalix[6]arene nitrile derivatives (3 and 4). The trinitrile derivative 3 has been synthesized from 5,11,17,23,29,35‐hexa‐tert‐butyl‐37,38,39,40,41,42‐hexahydroxycalix[6]arene 1. The compound 1 was directly converted to its hexanitrile derivative 4. In these syntheses, it was thought to explore the role of nitrile sites in the extraction of various metal cations and HCr2O7 ?/Cr2O7 2? anions. The liquid‐liquid extraction properties of 3 and 4 towards selected alkali/transition metal cations and HCr2O7 ?/Cr2O7 2? anions are reported. It has been observed that receptor 3 does not extract alkali/transition metal cations effectively, but shows affinity towards HCr2O7 ?/Cr2O7 2? anions at low pH. The compound 4 is an effective form for transferring Ni2+ selectively, while it shows poor transferring ability for HCr2O7 ?/Cr2O7 2? anions from an aqueous into a dichloromethane layer. It was observed that the cavity size of the calix[n]arenes and the cooperativity of the functionalities play important roles in two phase extraction systems. 相似文献
938.
Electrically conductive composite systems based on polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA) filled with metal powders of Al and Cu have been studied. The composite preparation conditions allow the formation of a random distribution of metallic particles in the polymer matrix. Dependence of the dielectric and conductivity properties of the PVC and PMMA/fillers was studied over a broad range of frequency and volume fraction of metal fillers. The experimental results could be explained by means of the conductivity of fillers and the interface polarization between polymers and fillers. Percolation was also seen in this study when the volume fraction of conducting fillers was close to critical value, in which the composites undergo an insulator‐conductor transition. The relation among the dielectric property and the fillers with different conductivity was proposed. 相似文献
939.
Ligand bridged polymeric complexes of the type [M(apainh)(H2O)X] where, M=Mn(II), Co(II), Ni(II), Cu(II), and Zn(II); X=Cl2 or SO4; apainh=acetone p‐amino acetophenone isonicotinoyl hydrazone have been synthesized and characterized. The complexes are stable solids, insoluble in common organic solvents and are non‐electrolytes. Magnetic moments and electronic spectral studies suggest a spin‐free octahedral geometry for all Mn(II), Co(II), Ni(II), and Cu(II) complexes. IR spectra show tridentate nature of the ligand bonding through two >C?N and a >C?O groups. X‐ray powder diffraction parameters for some of the complexes correspond to orthorhombic and tetragonal crystal lattices. Thermal studies (TGA and DTA) of [Mn(apainh)(H2O)SO4] complex show multi‐step decomposition pattern of both an endothermic and exothermic nature. ESR data of Cu(II) chloride complex in solid state show an axial spectra, whereas, Cu(II) sulfate complex is isotropic in nature. The complexes show a significant antifungal activity against a number of pathogenic fungal species and antibacterial activity against Pseudomonas sp. and Clostridium sp. The metal complexes are more active than the ligand. 相似文献
940.
Nives Galić Nataša Burić Renato Tomaš Leo Frkanec Vladislav Tomišić 《Supramolecular chemistry》2013,25(5):389-397
The fluorescent peptidocalixarenes, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (1) and 5,11,17,23-tetra-tert-butyl-25,27-di(O-methyl)-26,28-bis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (2), were prepared by introducing tryptophan subunits at a lower calixarene rim. Coordination abilities of 1 and 2 towards Eu(III) and alkali metal cations were studied by spectrophotometric, spectrofluorimetric, conductometric and potentiometric titrations in acetonitrile at 25°C. Rather strong complexation was observed for smaller alkali metal cations Li+ and Na+ (log K Li1 >6, log K Li2 >6, log K Na1 = 8.25, log K Na2 = 6.94), and moderate for K+ (log K K1 = 5.09, log K K2 = 4.09). Larger Rb+ and Cs+ cations did not fit in the ion binding site of 1 so no complexation was detected, whereas the more flexible ligand 2 accommodated Rb+ cation (log K Rb2 = 3.44). The fluorescence of 1 (λex = 280 nm, λem = 340 nm) was remarkably quenched by Eu(III). Stability constant of 1:1 (Eu3+:1) complex determined spectrofluorimetrically amounted to log K Eu1 = 6.16. 相似文献