全文获取类型
收费全文 | 3361篇 |
免费 | 154篇 |
国内免费 | 83篇 |
专业分类
化学 | 2500篇 |
晶体学 | 15篇 |
力学 | 177篇 |
数学 | 394篇 |
物理学 | 512篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 67篇 |
2021年 | 91篇 |
2020年 | 119篇 |
2019年 | 149篇 |
2018年 | 203篇 |
2017年 | 151篇 |
2016年 | 271篇 |
2015年 | 167篇 |
2014年 | 237篇 |
2013年 | 449篇 |
2012年 | 284篇 |
2011年 | 255篇 |
2010年 | 203篇 |
2009年 | 169篇 |
2008年 | 183篇 |
2007年 | 138篇 |
2006年 | 74篇 |
2005年 | 70篇 |
2004年 | 74篇 |
2003年 | 53篇 |
2002年 | 41篇 |
2001年 | 17篇 |
2000年 | 19篇 |
1999年 | 17篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1963年 | 1篇 |
排序方式: 共有3598条查询结果,搜索用时 15 毫秒
61.
Vahideh Masoumi Ali Mohammadi Mohsen Amini Mohammad Reza Khoshayand Rasoul Dinarvand 《Journal of Solid State Electrochemistry》2014,18(6):1763-1771
In this work, polyaniline, polypyrrole, and polyaniline/polypyrrole composite fibers were synthesized in the absence and presence of oxidized multiwalled carbon nanotubes using electrochemical cyclic voltammetry with CF3COOH as dopant. Thermal stability of these fibers was studied by differential scanning calorimetry. Then, headspace solid-phase microextraction process coupled with gas chromatography and flame ionization detector was used for comparing extraction capability of benzaldehyde from aqueous solution. Since polyaniline fiber showed better extraction efficiency than the other fibers, its preparation conditions including acid concentration, aniline concentration, scan rate, and amount of multiwalled carbon nanotubes were studied by means of the “one-factor-at-a-time method”. The analytical performance of polyaniline fibers were investigated to determine benzaldehyde from the aqueous solution. The morphology and texture of polyaniline fibers were examined by field emission scanning electron microscopy and Fourier transform infrared spectroscopy analyses. The attained results revealed that the perfect conditions for acid concentration, aniline concentration, scan rate, and multiwalled carbon nanotubes content were 0.5 M, 0.2 M, 25 mV s?1, and 0.02 wt%, respectively. The limit of detection for the proposed polyaniline fiber was 15 ng ml?1. 相似文献
62.
Back Cover: Quantum Mechanics/Molecular Mechanics Study on the Oxygen Binding and Substrate Hydroxylation Step in AlkB Repair Enzymes (Chem. Eur. J. 2/2014) 下载免费PDF全文
63.
Electrochemically Generated Nanoparticles of Halogen‐Bridged Mixed‐Valence Binuclear Metal Complex Chains 下载免费PDF全文
Emiliano Martínez‐Periñán Mohammad‐Reza Azani Dr. José M. Abad Dr. Eva Mateo‐Martí Dr. Félix Pariente Dr. Rubén Mas‐Ballesté Dr. Félix Zamora Prof. Encarnación Lorenzo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(23):7107-7115
Spherical nanoparticles composed of MMX chains can be made by a polymerization strategy driven by electrochemical processes. In particular, the [Pt2(MeCS2)4I2] (MMI2) dimetal subunit is employed as a monomer for the formation of [Pt2(MeCS2)4I]n spherical nanostructures on surfaces. We have paid particular attention to elucidating the general mechanism of the deposition process on the basis of in situ electrochemical measurements. The reduction of MMI2 to give the electrodeposition of nanostructures agrees well with formation of the reduced [MMI2]? species followed by a disproportionation mechanism mediated by iodide anions. The chemical composition of the particles was determined by energy‐dispersive X‐ray spectroscopy (EDX) and X‐ray photoelectron spectroscopy (XPS) to reveal the MMI2 polymer. 相似文献
64.
Keypour Hassan Liyaghati-Delshad Mozhdeh Rezaeivala Maji Khavasi Hamid Reza 《无机化学学报》2014,30(7):1733-1740
在Mn髤模板作用下,2,6-diacetylpyridine(DAP)及合适链状胺化合物通过[1+1]环缩合反应,合成了3个大环席夫碱配合物,并进行了红外、元素分析、质谱及电导率等表征及研究。测得了配合物[MnL1(CH3CN)](ClO4)2的晶体结构,中心离子呈现出稍微扭曲的五角锥配位构型。研究了配合物对S.aureus(ATCC 6633),B.cereus(ATCC 7064),C.xerosis(ATCC 373)(gram-positive bacterial strains),E.coli(PTCC 10009),K.pneuomoniae(MTCC 109),and P.vulgaris(lio)(gram-negative bacterial strains)的抗菌活性。结果显示[MnL3](ClO4)2抗菌活性明显优于[MnL1(CH3CN)](ClO4)2and[MnL2](ClO4)2。在25℃条件下0.1 mol·L-1 KCl溶液中,通过电位计量法测定了化合物的质子化常数。 相似文献
65.
Ali Akbarzadeh Seyed Omid Ranaei Siadat Mostafa Motallebi Mohammad Reza Zamani Mohammad Barshan Tashnizi Sakineh Moshtaghi 《Applied biochemistry and biotechnology》2014,172(4):2253-2265
Bioconversion of cellulosic material into glucose needs cellulase enzymes. One of the most important organisms that produces cellulases is Trichoderma reesei, whose cellulose enzymes are probably the most widely used in the industry. However, these enzymes are not stable enough at high pH and temperatures. The optimized synthetic endoglucanase II gene with Pichia pastoris codon preferences was secretary expressed in P. pastoris. Recombinant enzyme characterization showed maximum activity at pH 4.8 and temperature 75 °C, and it demonstrated increasing thermal stability in high temperature. The enzyme maintained its activity in a wide pH range from 3.5 to 6.5. The optimization of fermentation medium was carried out in shaking flasks. Recombinant protein expression at optimum conditions (pH 7, temperature 25 °C, and 1 % methanol induction) for 72 h demonstrated 2,358.8 U/ml endoglucanase activity units. To our knowledge, this is the highest acidic thermophilic endoglucanase activity that is reported in crude intracellular medium in P. pastoris. We conclude that P. pastoris is an appropriate host for high-level expression of optimized endoglucanase gene with improved thermal stability. 相似文献
66.
The relationship between the rheological properties of nylon-6,6 solutions and the morphology of their electrospun nanofibers was established. The viscosity of nylon-6,6 in formic acid(90%) was measured in the concentration range of 5 wt%-25 wt% using a programmable viscometer. Electrospinning of nylon-6,6 solutions was carried out under controlled parameters. The chemical structure, morphology and thermal properties of the obtained nanofibers were investigated using Fourier transform infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and differential scanning calorimetry(DSC), respectively. Entanglement concentration(ce) was found to be 15 wt% and a power law relationship between specific viscosity and solution concentration was observed with exponents of 2.0 and 3.3 for semi-dilute unentangled(c ce) and semi-dilute entangled(c ce) regimes, respectively. The diameter and uniformity of the nanofibers were found to be dependent on the viscosity. Moreover, the average diameter of electrospun nanofibers was found to be dependent on zero shear rate viscosity and normalized concentration(c/ce) in a power law relationship with exponents of 0.298 and 0.816, respectively. For nylon-6,6 solutions, the entanglement concentration(ce = 15 wt%) provides the threshold viscosity required for the formation of a stable polymeric jet during electrospinning and producing uniform beadless fibers. For concentrations less than ce, beaded fibers with some irregularities are formed. DSC analysis showed an increase in crystallinity of all electrospun samples compared to original polymer. Furthermore, Based on FTIR spectroscopy, α phase is dominant in electrospun nanofibers and minor amount of β and γ phases is also available. 相似文献
67.
A convenient and efficient solvent-free procedure is described for preparation of 6-amino-4-aryl-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles by four-component reaction of hydrazine hydrate, ethyl acetoacetate, aryl aldehyde, and malononitrile in the presence of a catalytic amount of titanium dioxide nano-sized particles. Short reaction times, high yields under ambient conditions, simple reaction, clean work-up, and reusability of the nano heterogeneous catalyst are the advantages of this method. 相似文献
68.
A new, green, and highly efficient protocol for the expeditious preparation of some α,α′-bis[(aryl or allyl)idene]cycloalkanones and 2-[(aryl or allyl)idene]-1-indanones via a simple microwave-assisted Claisen–Schmidt condensation reaction catalyzed by MoCl5 was successfully developed. Outstanding features of the current methodology include the use of solvent-free conditions, simple operation, use of a very inexpensive and available catalyst, low catalyst loading, short reaction times, high yields of the pure products, no harmful by-products, easy workup, and also the applicability of microwave irradiation as a clean source of energy. Furthermore, a gram-scale reaction was successfully conducted, proving the scalability of this current Claisen–Schmidt condensation reaction. 相似文献
69.
Substituent effect on the structure and electronic properties of H2Si=SiHAr (Ar = p-C6H4X; X = NH2, OH, Me, H, F, Cl, CHO, COOH, CN, NO2) molecules are studied at the CAM-B3LYP/6-311G(d,p) level of theory. Energy decomposition analysis (EDA) is used as a useful tool for illustrating the interaction between H2Si and SiHAr fragments in HArSi=SiH2 molecules. Energetic analysis reveals that the singlet state of the fragments is more stable than triplet state. Also, interactions are stronger in the presence of electron-withdrawing groups (EWGs) in comparison to electron donating groups (EDGs). EDG and EDG effects are investigated on the stability of fragments, frontier orbital energy, distortion, HOMO–LUMO gap, electron-donating (ω−) and electron-accepting (ω+) powers of the studied molecules. Then, the correlations between these calculated parameters with the Hammett and Brown constants (σp and σp+, respectively) are provided. Also, time-dependent density functional theory method (TD-DFT) is employed for the determination of the strongest absorption band values (λmax,el) of these molecules. This absorption band is attributed to the HOMO →LUMO transition. 相似文献
70.
Mitra Ebrahimi Shahrzad Abdolmohammadi Reza Kia-Kojoori 《Journal of heterocyclic chemistry》2020,57(4):1875-1881
Ag2Cr2O7 nanoparticles were found to be an exceedingly effective catalyst for the mild and green synthesis of aminobenzochromenes. The reaction was performed under ultrasonic irradiation as an innocuous tool and in water as a green solvent at room temperature. This method encompasses several sustainable and economic benefits. 相似文献