全文获取类型
收费全文 | 26244篇 |
免费 | 4310篇 |
国内免费 | 4017篇 |
专业分类
化学 | 20387篇 |
晶体学 | 424篇 |
力学 | 1280篇 |
综合类 | 296篇 |
数学 | 2948篇 |
物理学 | 9236篇 |
出版年
2024年 | 63篇 |
2023年 | 464篇 |
2022年 | 790篇 |
2021年 | 927篇 |
2020年 | 1042篇 |
2019年 | 1106篇 |
2018年 | 913篇 |
2017年 | 963篇 |
2016年 | 1251篇 |
2015年 | 1267篇 |
2014年 | 1613篇 |
2013年 | 2025篇 |
2012年 | 2425篇 |
2011年 | 2539篇 |
2010年 | 1945篇 |
2009年 | 1836篇 |
2008年 | 1947篇 |
2007年 | 1802篇 |
2006年 | 1684篇 |
2005年 | 1345篇 |
2004年 | 987篇 |
2003年 | 783篇 |
2002年 | 740篇 |
2001年 | 651篇 |
2000年 | 573篇 |
1999年 | 481篇 |
1998年 | 352篇 |
1997年 | 312篇 |
1996年 | 266篇 |
1995年 | 254篇 |
1994年 | 233篇 |
1993年 | 168篇 |
1992年 | 145篇 |
1991年 | 135篇 |
1990年 | 121篇 |
1989年 | 89篇 |
1988年 | 73篇 |
1987年 | 53篇 |
1986年 | 60篇 |
1985年 | 49篇 |
1984年 | 18篇 |
1983年 | 18篇 |
1982年 | 18篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1971年 | 4篇 |
1957年 | 4篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
241.
Shujiang Tu Bo Jiang Hong Jiang Yan Zhang Runhong Jia Junyong Zhang Qingqing Shao Chunmei Li Dianxiang Zhou Longji Cao 《Tetrahedron》2007,63(25):5406-5414
Reaction of arylidenemalononitriles, 1,3-indanedione, and mercaptoacetic acid or 4-methylbenzenethiol is successfully carried out using microwave heating. It is an efficient and promising synthetic strategy to build the indenopyridine skeleton. 相似文献
242.
Preparation of flame retardant polyamide 6 composite with melamine cyanurate nanoparticles in situ formed in extrusion process 总被引:1,自引:0,他引:1
This paper is focused on in situ preparation of melamine cyanurate (MCA) nanoparticles from reaction of melamine (MEL) and cyanuric acid (CA) and their flame retardant polyamide 6 (PA6) composite in the extrusion process through a novel reactive processing method. Fourier transform infrared (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were utilized to characterize the in situ formed MCA nanoparticles and their blends with PA6. Introduction of pentaerythritol (LTP) and water-bound plasticizer dioctyl phthalate (DPT) into the extrusion reaction system greatly inhibits the evaporation of water required for melamine and cyanuric acid reaction at high temperature (higher than 180 °C), laying a foundation for successful in situ preparation of MCA through reactive processing. XRD and FT-IR measurements indicate that under the effect of pentaerythritol, dioctyl phthalate and water, melamine really reacts with cyanuric acid to in situ form MCA in extrusion process. The reaction degree is close to 100%. A very important finding through SEM is that the in situ formed MCA particles, which were found to have aspect ratio of about 7.5, radial size in the range of 70-300 nm (mostly 70-90 nm) and crystallite size of less than 22 nm, are uniformly dispersed in the matrix PA6 at nanoscale. The in situ formed MCA nanoparticles greatly improve the flame retardancy and the mechanical properties of flame-retarded PA6 materials, and the introduced plasticizer dioctyl phthalate also ameliorates the related impact property. The obtained flame-retarded PA6 materials have good comprehensive performance with flame retardancy UL-94 V-0 rating at 1.6 and 3.2 mm thickness, tensile strength 48.0 MPa, elongation at break 106.3% and Izod notched impact strength 8.92 kJ/m2. Compared with flame-retarded PA6 material with in situ formed MCA, the one prepared through conventional blending of PA6 with commercial MCA product has improved tensile strength but deteriorated impact strength and flame retardancy. 相似文献
243.
In this paper, the spectral behavior of protein and Poniacyl Carmine 2B (PC 2B) has been studied by spectrophotometric method. The conditional constants, apparent combination constant K and maximum binding number n, were used to express the combination ability of the reactions between PC 2B and protein under a set of given conditions. The Sandell index s was used to express the sensitivity of the determination of protein. The factors, acidity, PC 2B concentration and the ionic strength, were discussed by the change of apparent combination constant and maximum binding number. It was found that acidity of the solution, PC 2B concentration and ionic strength had a significant effect on the sensitivity of the assay of protein. Under the optimal conditions, the apparent combination constant K and the maximum binding number n were 2.36 × 106 L mol?1 and 95, respectively. With further investigation, it was found that the Scatchard model was suitable in treating the data obtained in the experiments. In the buffer medium of HCl‐KCl at 1.87, the addition of protein made the maximum absorption of the system move from 527 nm to 513 nm. Its apparent molar absorptivity is 4.46 × 105 Lmol?1 cm?1 at 513 nm. Beer's law is obeyed in the range of 0 ? 55 μg mL?1. The system developed in this paper has been used for the determination of protein in milk powder successfully. 相似文献
244.
Xiao-pingYan Bao-linHe JieZhang Han-fanLiua 《高分子科学》2005,(4):393-399
Modification of transition metal cations to polymer-stabilized Pt colloidal clusters modified with cinchonidine was studied in enantioselective hydrogenation of methyl pyruvate. Compared to the enantiomeric excess (e.e.) value (71.4%) obtained without the presence of metal cations, obvious e.e. enhancement (up to 82.5%) was resulted from the addition of Zn^2 but with a certain decrease in activity. The reaction parameters in the presence of Zn^2 were also studied. It was found that the Pt colloidal catalysts in the presence of metal cations performed very differently from that in the absence of metal cations. 相似文献
245.
246.
Synthesis of Polyacrylaminothiourea Chelating Fiber and Properties of Concentration and Separation of Trace Noble Metal Ions from Samples 总被引:1,自引:0,他引:1
A novel polyacrylaminothiourea chelating fiber was synthesized simply and rapidly from nitrilon (an acrylonitrile-based synthetic fiber), which was applied to preconcentrate and separate of trace amount of Au(Ⅲ),Pt(Ⅳ),Pd(Ⅳ) and Ir (Ⅳ) ions from solution of samples.The analyzed ions can be quantitatively concentrated by the fiber up to a flow rate of 20.0mL/min at pH2, and can also be desorbed with 15 mL of 4mol/L HCl 3% thiourea from the fiber column with recoveries of 96.5%-100%.The chelating fiber can be reused for ten times,the recoveries of these ions are still over 92%,and hundred to thousand times of excess of Fe(Ⅲ),Al(Ⅲ),Ca(Ⅱ),Mg(Ⅱ),Ni(Ⅱ),Mn(Ⅱ),Cu(Ⅱ),Zn(Ⅱ),and Cd(Ⅱ) cause no interference on the determination of the analyzed ions by inductively-coupled plasma atomic emission spectrometry (ICP-AES).The static saturation adsorption capacities of the fiber for the analytes are in the range of 1.15-2.80mmol/g.The relative standard deviations for the determination of 20.0ng/mL each of Au(Ⅲ),Pt(Ⅳ),Pd(Ⅳ)and Ir(Ⅳ) are in the range of 0.7%-3.0%.The recoveries for test from standard additions to real solution samples are between 96% and 100%.The concentration of each ion in powder sample detected by the method is in good agreement with the certified value. 相似文献
247.
248.
在现有方法基础上对沉积物中磺胺类、喹诺酮类、大环内脂类和四环素类抗生素的提取、富集、净化以及仪器分析方法进行了优化。以EDTA-Mcllvaine缓冲溶液与乙腈(V:V,1:1)混合液作为提取液,利用超声波细胞破碎仪进行超声提取,串联强阴离子交换柱(SAX)和HLB固相萃取柱进行固相萃取(SPE),通过超高效液相/串联质谱(UPLC-MS/MS)测定沉积物中抗生素的含量。抗生素基质加标回收率在56.4%~110%,相对标准偏差为1.1%~24.3%,方法检出限0.0055~0.716 ng/g。本方法有效地提高了沉积物中抗生素的提取效率,并应用于实际样品的测定中。 相似文献
249.
Three new 8-hydroxyquinoline derivatives, i.e. 5-[(4-styryl-benzylidene)-amino]-quinolin-8-ol (1), 5-[(4-bromo-2-fluoro-benzylidene)-amino]-quinoline-8-ol (2) and 2-[2-(9-ethyl-9H-carbazol-2yl)-vinyl]-quinolin-8-ol (3), and their metallic complexes were synthesized and identified by ultraviolet-visible (UV-Vis), 1H nuclear magnetic resonance (1H NMR), Fourier transform infrared spectrometer (FTIR), mass spectrometry (MS) spectra and elemental analyses. Their fluorescence
properties were studied by photoluminescence, which indicated that the luminescence wavelength of 5-and 2-substitued-8-hydroxyquinoline
derivatives shifted to red in comparison with that of 8-hydroxyquinoline. Meanwhile, the fluorescence lifetime of 2-[2-(9-ethyl-9H-carbazol-2yl)-vinyl]-quinolin-8-ol
and its zinc complex showed long lifetime in benzene solution.
__________
Translated from Chinese Journal of Organic Chemistry, 2007, 27(3): 402–408 [译自: 有机化学] 相似文献
250.
The harmonic vibrational force fields and the IR spectrum of XSO2NCO (X= F, C1) molecules have been studied usingab initio HF/SCF method with the 6-31G’ basis set. Theab initio harmonic force fields are scaled empirically using the scaled quantum mechanical (SQM) method of Pulay. A set of scale factors
are optimized by the least-squares fitting to the experimental frequencies of FSO2NCO and then are transferred to CISO2NCO to give ana priori prediction of its fundamental frequencies. The average deviations between the theoretical frequencies and the experimental
values for FSO2NCO and C1SO2NCO are 3 and 5 cm-1, respectively. The assignments of the fundamentals for these two molecules are also made atcording to the potential energy
distributions and theab initio IR intensities
Project supported by the National Natural Science Foundation of China (Grant No. 29673029) 相似文献