全文获取类型
收费全文 | 44153篇 |
免费 | 5556篇 |
国内免费 | 2232篇 |
专业分类
化学 | 5251篇 |
晶体学 | 68篇 |
力学 | 3030篇 |
综合类 | 596篇 |
数学 | 9933篇 |
物理学 | 9541篇 |
无线电 | 23522篇 |
出版年
2024年 | 112篇 |
2023年 | 513篇 |
2022年 | 815篇 |
2021年 | 1184篇 |
2020年 | 1324篇 |
2019年 | 852篇 |
2018年 | 858篇 |
2017年 | 1392篇 |
2016年 | 1717篇 |
2015年 | 1785篇 |
2014年 | 3073篇 |
2013年 | 2870篇 |
2012年 | 3123篇 |
2011年 | 3114篇 |
2010年 | 2364篇 |
2009年 | 2480篇 |
2008年 | 2922篇 |
2007年 | 2944篇 |
2006年 | 2656篇 |
2005年 | 2325篇 |
2004年 | 2119篇 |
2003年 | 1927篇 |
2002年 | 1604篇 |
2001年 | 1244篇 |
2000年 | 1031篇 |
1999年 | 919篇 |
1998年 | 747篇 |
1997年 | 650篇 |
1996年 | 575篇 |
1995年 | 584篇 |
1994年 | 384篇 |
1993年 | 342篇 |
1992年 | 255篇 |
1991年 | 243篇 |
1990年 | 147篇 |
1989年 | 103篇 |
1988年 | 128篇 |
1987年 | 82篇 |
1986年 | 67篇 |
1985年 | 69篇 |
1984年 | 58篇 |
1983年 | 38篇 |
1982年 | 34篇 |
1981年 | 39篇 |
1980年 | 27篇 |
1979年 | 30篇 |
1978年 | 16篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1957年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
G.B. Song C.Q. Han J.K. Liang Q.L. Liu F.S. Liu G.H. Rao 《Journal of solid state chemistry》2004,177(7):2394-2403
The subsolidus phase relations of the PrOx-CaO-CuO pseudo-ternary system sintered at 950-1000°C have been investigated by X-ray powder diffraction. In this system, there exist one compound Ca10Pr4Cu24O41, one Ca2Pr2Cu5O10-based solid solution, seven three-phase regions and two two-phase regions. The crystal structures of Ca10Pr4Cu24O41 and Ca2Pr2Cu5O10-based solid solution have been determined. Compound Ca10Pr4Cu24O41 crystallizes in an orthorhombic cell with space group D2h20−Cccm, Z=4. Its lattice parameters are a=11.278(2) Å, b=12.448(3) Å and c=27.486(8) Å. The crystal structure of Ca2Pr2Cu5O10-based solid solution is an incommensurate phase based on the orthorhombic NaCuO2 type subcell. The lattice parameters of the subcell of the Ca2.4Pr1.6Cu5O10 are a0=2.8246(7) Å, b0=6.3693(5) Å, c0=10.679(1) Å, and those of the orthorhombic superstructure are with a=5a0, b=b0, c=5c0. The Ca2.4Pr1.6Cu5O10 structure can also be determined by using a monoclinic supercell with space group C2h5−P21/c, Z=4, a=5a0, b=b0, and β=104.79(1)° or 136.60(1)°, V=5a0b0c0. 相似文献
902.
A search system is presented that utilizes Fourier domain data. This system removes dominating features from the infrared absorbance data, transforms the data into the Fourier domain and performs a complete library search. The actual matching algorithm is very simple and excellent search results were attained using 16 cm–1 resolution infrared absorbance spectra before transformation. This method can be used to distinguish very similar spectra that cannot be distinguished by many other search methods. 相似文献
903.
Óscar López 《Tetrahedron》2005,61(38):9058-9069
An efficient method for the preparation of urea-bridged cyclodextrins using triphosgene in the isocyanation step in an aqueous two-phase system is reported. Per-O-acetylated glycopyranosylamines and 2-amino-2-deoxy-α and β-d-glucose were also transformed into the corresponding isocyanates using either an aqueous two-phase or an anhydrous dichloromethane medium, and converted in situ into ureas. An alternative method for the preparation of sugar-derived ureas consisting of desulfurization of sugar thioureas with mercury oxide is also presented. 相似文献
904.
阐述了ICP-AES专家系统中AES电离和激发模型的基本原理,研究了模拟中应用non-LTE理论的正确性以及所需的支持数据库。利用Boltzmann曲线,讨论了Boltzmann温度的测量值随能级激发能变化的现象,并与类似条件下的经验Boltzmann曲线进行对比,证明了专家系统中AES电脑和激发模型可靠性。 相似文献
905.
Roxana Irimescu 《Tetrahedron letters》2004,45(3):523-525
Lipase-catalyzed enantioselective acylation of 1-phenylethylamine and 2-phenyl-1-propylamine was performed by reacting the amines with carboxylic acids in a non-solvent system or in ionic liquids as reaction media. The reaction equilibrium was shifted toward amide synthesis by the removal of formed water under reduced pressure. 相似文献
906.
907.
Wenjuan Yin Shihai Yan Mei Qin Zhiqiang Li Yuxiang Bu 《International journal of quantum chemistry》2006,106(7):1528-1543
In this work, a density function theory (DFT) study is presented for the HNS/HSN isomerization assisted by 1–4 water molecules on the singlet state potential energy surface (PES). Two modes are considered to model the catalytic effect of these water molecules: (i) water molecule(s) participate directly in forming a proton transfer loop with HNS/HSN species, and (ii) water molecules are out of loop (referred to as out‐of‐loop waters) to assist the proton transfer. In the first mode, for the monohydration mechanism, the heat of reaction is 21.55 kcal · mol?1 at the B3LYP/6‐311++G** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal · mol?1 compared with the no‐water‐assisting isomerization barrier T (65.52/43.87 kcal · mol?1). But when adding one water molecule on the HNS, there is another special proton‐transfer isomerization pathway with a transition state 10T′ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal · mol?1. Clearly, this process is more difficult to follow than the R–T–P process. For the two‐water‐assisting mechanism, the heat of reaction is 19.61 kcal · mol?1, and the forward/backward barriers are 32.27/12.66 kcal · mol?1, decreased by 33.25/31.21 kcal · mol?1 compared with T. For trihydration and tetrahydration, the forward/backward barriers decrease as 32.00/12.60 (30T) and 37.38/17.26 (40T) kcal · mol?1, and the heat of reaction decreases by 19.39 and 19.23 kcal · mol?1, compared with T, respectively. But, when four water molecules are involved in the reactant loop, the corresponding energy aspects increase compared with those of the trihydration. The forward/backward barriers are increased by 5.38 and 4.66 kcal · mol?1 than the trihydration situation. In the second mode, the outer‐sphere water effect from the other water molecules directly H‐bonded to the loop is considered. When one to three water molecules attach to the looped water in one‐water in‐loop‐assisting proton transfer isomerization, their effects on the three energies are small, and the deviations are not more than 3 kcal · mol?1 compared with the original monohydration‐assisting case. When adding one or two water molecules on the dihydration‐assisting mechanism, and increasing one water molecule on the trihydration, the corresponding energies also are not obviously changed. The results indicate that the forward/backward barriers for the three in‐loop water‐assisting case are the lowest, and the surrounding water molecules (out‐of‐loop) yield only a small effect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
908.
909.
烯烃氢甲酰化反应研究进展 总被引:4,自引:1,他引:4
本文综述了烯烃氢甲酰化反应的催化剂从钴配合物到铑配合物和从均相催化体系到两相催化体系的发展过程。比较了三种催化剂和两个催化体系的特点、应用情况及研究工作进展. 相似文献
910.
"Living"/controlled radical polymerization of styrene was carried out with diethyl 2,3-dicyano-2,3-diphenylsuccinate (DCDPS)/CuCl2/bipyridine (bipy) initiation system at 120℃. The molecular weights of resultant PSt increased with the monomer conversion and the polydispersities were in the range of 1.37 ~ 1.52. A linear ln([M]o/[M])versus time plot was also obtained indicating the constant concentration of growing radicals during the polymerization with this initiation system. End group analysis by 1H-NMR spectroscopic studies showed that the end groups of the polymer obtained is cω-functionalized by a chlorine group from the catalyst and a-functionalized by a (carbethoxy-cyano-phenyl)methyl group from the fragments of the initiator. Having C1 atom at the chain end, the PSt obtained can be used as a macroinitiator to promote a chain-extension reaction with fresh St and block copolymerization reaction with a second monomer, such as methyl methacrylate, in the presence of CuC1/bipy catalyst via a conventional ATRP process. 相似文献