全文获取类型
收费全文 | 23727篇 |
免费 | 3788篇 |
国内免费 | 2162篇 |
专业分类
化学 | 6986篇 |
晶体学 | 74篇 |
力学 | 2961篇 |
综合类 | 606篇 |
数学 | 10215篇 |
物理学 | 8835篇 |
出版年
2024年 | 37篇 |
2023年 | 240篇 |
2022年 | 379篇 |
2021年 | 553篇 |
2020年 | 606篇 |
2019年 | 563篇 |
2018年 | 549篇 |
2017年 | 833篇 |
2016年 | 950篇 |
2015年 | 764篇 |
2014年 | 1248篇 |
2013年 | 1620篇 |
2012年 | 1396篇 |
2011年 | 1483篇 |
2010年 | 1311篇 |
2009年 | 1486篇 |
2008年 | 1726篇 |
2007年 | 1762篇 |
2006年 | 1460篇 |
2005年 | 1344篇 |
2004年 | 1279篇 |
2003年 | 1064篇 |
2002年 | 986篇 |
2001年 | 817篇 |
2000年 | 707篇 |
1999年 | 668篇 |
1998年 | 578篇 |
1997年 | 493篇 |
1996年 | 443篇 |
1995年 | 434篇 |
1994年 | 296篇 |
1993年 | 272篇 |
1992年 | 202篇 |
1991年 | 240篇 |
1990年 | 149篇 |
1989年 | 95篇 |
1988年 | 121篇 |
1987年 | 81篇 |
1986年 | 67篇 |
1985年 | 68篇 |
1984年 | 51篇 |
1983年 | 37篇 |
1982年 | 42篇 |
1981年 | 45篇 |
1980年 | 28篇 |
1979年 | 30篇 |
1978年 | 19篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1974年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A. B. A. Elgazzar A. M. Gafaar H. N. Hafez A. S. Aly 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):1859-1883
Pyridopyrimidine derivatives 2 reacted with hydrazonoylchloride derivatives and yielded triazolopyridopyrimidines 6a–f. Compound 4b reacted with aliphatic acids and afforded triazolo-pyridopyrimidines 7a,b, and the reaction with carbon disulfide afforded 10-mercapto-triazolopyridopyrimidine (10). Moreover, the reaction of 4b with β -ketoesters afforded 10-pyrazolyl-pyridopyrimidines derivatives 11, 13, 14, and 15. Compound 4b reacted with nitrous acid to give tetrazolopyridopyrimidine 16, which reduced to 10-amino-derivative 17. On the other hand, the reaction of 4b with aromatic aldehydes afforded arylidines derivatives 18a–c, which were later cyclized to triazolo-pyridopyrimidines deivatives 19a–c. Finally, 4b reacted with α-haloketones to give triazines derivativrs 20, with new ring systems. 相似文献
992.
Majid M. Heravi Fereshteh Hydarzadeh Yahya Farhangi Mitra Ghassemzadeh 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):1473-1475
Primary and secondary trimethylsilyl ethers are effeciently converted to the corresponding carbonyl compounds using HZSM-5 zeolite-supported CrO 3 under microwave irradiation in solventless system. 相似文献
993.
Svetlana F. Malysheva Natal’ya A. Belogorlova Nina K. Gusarova Alexander V. Artem’ev Alexander I. Albanov Boris A. Trofimov 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):1688-1693
Abstract Red phosphorus reacts with allylbenzene in the superbase system KOH-DMSO (130°C, 3 h, Ar) to give a mixture of bis(1-methyl-2-phenylethyl)phosphane (1), bis(1-methyl-2-phenylethyl)phosphane oxide (2), and 1-methyl-2-phenylethylphosphinic acid (3). Secondary phosphane oxide 2 and phosphinic acid 3 have been isolated from this mixture in 35% and 32% yield, respectively. Microwave activation of the reaction (200 W, 30 min) affords secondary phosphane 1 as the main product in 48% yield. GRAPHICAL ABSTRACT 相似文献
994.
The surface contact disinfecting technique is a newly developed method for water sterilization. In this paper, the grafted quaternary ammonium salts (QAS) antibacterial fibers were prepared and designed to apply for the surface contact disinfecting process in water treatment. The antibacterial fibers were directly prepared by grafting methacryloxylethyl benzyl dimethyl ammonium chloride (DMAE-BC) onto cellulose fiber using thiocarbonate-H2O2 redox system. All kinds of factors in the grafting reactions, such as reaction time, reaction temperature, monomer concentration, initiator concentration, which influence the percentage of grafting, were studied and optimized. The modified cellulose fibers were characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope(SEM). The effects of the percentage of grafting of the grafted cellulose fibers on bactericidal activity were also studied. The spread plate method was used to characterize the bactericidal activity. The disinfection process was further investigated by directly observing the morphology of the bacterial cells adsorbed on the antibacterial fibers with SEM and measuring extracelluar total protein concentration in suspension. The poly(DMAE-BC)-grafted cellulose ?ber was found to exhibit particularly high activity against E.coli. 相似文献
995.
The present study deals with the modification of sterculia gum to develop the novel colon specific delivery system for use in colon cancer. The sterculia and acrylic acid based hydrogels were synthesized and characterized with FTIR, SEMs, TGA and swelling behavior. Swelling studies of the hydrogels were carried out as a function of reaction parameters such as monomer concentration, initiator concentration, amount of sterculia gum and crosslinker concentration and nature of swelling mediums. Swelling kinetics of the hydrogels and in vitro release dynamics of anticancer model drug methotrexate from the hydrogels were studied to evaluate the swelling mechanism and drug release mechanism from the drug-loaded hydrogels. The values of diffusion exponent for the release of drug were 0.883, 0.910 and 0.787 in distilled water, pH 2.2 buffer and pH 7.4 buffer, respectively. The release of drug from the polymer matrix occurred through a non -Fickian type diffusion mechanism. 相似文献
996.
A micelle-forming polymerizable surfactant monomer, octylphenol polyoxyethylene acrylate (OP-10-AC), was synthesized, and then OP-10-AC was copolymerized with acrylamide (AM) to form nonionic hydrophobically modified polyacrylamide P(AM/OP-10-AC) through micellar copolymerization. In the absence of surfactants, it was investigated in detail that the effect of mineral salts and temperature on steady rheological properties of P(AM/OP-10-AC) solutions and the effect of concentrations on reduced viscosity of P(AM/OP-10-AC) in a dilute solution. The results indicate that concentrations of the copolymer, mineral salts and temperature had a strong influence on shear viscosity of P(AM/OP-10-AC) solutions, and the trend of reduced viscosity of P(AM/OP-10-AC) solutions was distinctly different from polyacrylamide with increasing concentrations of testing solutions. In addition, it was also investigated that stress-relaxation behavior of an aqueous solution of P(AM/OP-10-AC)/KCl. As a result, a stress-relaxation model of the copolymer solutions was proposed, which can further verify the correctness of the conclusion on stress-relaxation behavior of hydrophobic association hydrogels in the paper reported previously. 相似文献
997.
In this paper, the diffusion mechanism of as‐spun PAN fiber was investigated in dimethyl sulfoxide‐water by determining the dynamic compositions of the fibers and the diffusion coefficients of solvent and nonsolvent during coagulation. The diffusion process could be divided into two stages. Results showed that the first stage of the diffusion process was the most important during the whole process, which was fundamental to further study on the formation mechanism. Also, compared with wet spinning, the dry‐jet wet spinning method had the advantage of mild coagulating at a high jet‐stretch. At high concentrations, the diffusion coefficients increased and the ratio of solvent diffusion coefficient to nonsolvent diffusion coefficient decreased; an increasing temperature resulted in the increase of both diffusion coefficients with a decrease in their ratios. To some extent, for the PAN‐DMSO‐water system, the more the ratios Ds*/Dn* tended to 1, the more the cross‐section shapes of as‐spun PAN fiber tended to be circular. 相似文献
998.
《高分子科学杂志,A辑:纯化学与应用化学》2013,50(12):1435-1450
ABSTRACT The kinetics of the free radical bulk polymerization of methyl methacrylate (MMA) was studied by DSC, using the benzoyl peroxide (BPO)/amine initiation system. N,N dimethyl-4-aminophenethyl alcohol (DMPOH), which is a newly synthesized and used amine in the preparation of acrylic dental resins and bone cements was examined, and the results compared to the most commonly used in these applications amine, the N,N dimethyl-p-toluidine (DMT). For both amines, the effect of the molar ratio of BPO/amine and of the reaction temperature, on the polymerization kinetics was investigated. The prepared polymers were characterized by determination of the average molecular weights (M¯ n and M¯ w ) and molecular weights distribution (M¯ w /M¯ n ) using Gel Permeation Chromatography. DMPOH was found to lead in slightly higher polymerization rates, lower gel times and lower molecular weights than DMT. The values of these parameters for both amines were influenced by the molar ratio of BPO to amine, when the product of the concentrations of these was kept constant. The highest polymerization rate occurred in the lowest gel time, resulting in polymers with the lowest molecular weight, and was observed when a molar ratio of about 1.5 BPO/amine was used. However, the final monomer conversion was found to be independent of the molar ratio and amine used. The activation energy of polymerization was found to be 51.8 kJ/mol K for BPO/DMPOH and 47.1 kJ/mol K for BPO/DMT. 相似文献
999.
Peng Zhang Wenzhe Wang Yingfang Zhou Gang Ruan Haiyang Yu Wenjuan Ji 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(11-12):764-769
AbstractA novel cationic water-soluble monomer allyldimethylisooctylammonium bromide (ADIAB) containing a short-chain alkane was synthesized successfully. This monomer was copolymerized with acrylamide and sodium acrylate to produce hydrophobically modified polyacrylamide (HMPAM) using solution polymerization without surfactants. The structures of monomer ADIAB and HMPAM were characterized with infrared spectroscopy and nuclear magnetic resonance spectroscopy. Influence of preparation condition on viscosities of products was studied. The aqueous solution viscosity of the terpolymer was also investigated as functions of concentration, temperature and salinity. The results showed that when the temperature exceeds the 60?°C and NaCl concentration exceeds about 2000?mg/L, the temperature and salt tolerance characters of terpolymer were demonstrated. The enhanced oil recovery tests were initially carried out using homogeneous sandpack models. 相似文献
1000.
Homogeneous blends of poly(L-lactide) (M n = 30 000 to 40 000) and poly(β-propiolactone) or poly(ε-caprolactone) were prepared in solution. The solvent-free blends were subjected to transesterification catalyzed by means of methyl triflate, triflic acid, boron trifluoride, or tributyltin methoxide at 100 or 150°C. At 100°C, transesterification was barely detectable even after 96 h. When poly(β-propiolactone) was used as the reactant at 150°C, degradation was faster than transesterification regardless of the catalyst. The same negative result was obtained for heterogeneous blends of poly(L-lactide) and poly(glycolide). In the case of poly(ε-caprolactone), copolyesters with slightly blocky sequences were obtained with tributyltin methoxide as catalyst, whereas the acidic catalysts caused rapid degradation. The copolyesters were characterized by means of 1H-NMR spectroscopy with regard to their molar composition, by means of 13C-NMR spectroscopy with regard to their sequences, and by means of differential scanning calorimetry with regard to crystallinity. 相似文献