全文获取类型
收费全文 | 15636篇 |
免费 | 1708篇 |
国内免费 | 2563篇 |
专业分类
化学 | 8794篇 |
晶体学 | 319篇 |
力学 | 935篇 |
综合类 | 48篇 |
数学 | 4661篇 |
物理学 | 5150篇 |
出版年
2024年 | 35篇 |
2023年 | 428篇 |
2022年 | 551篇 |
2021年 | 703篇 |
2020年 | 724篇 |
2019年 | 489篇 |
2018年 | 489篇 |
2017年 | 597篇 |
2016年 | 704篇 |
2015年 | 792篇 |
2014年 | 1080篇 |
2013年 | 1287篇 |
2012年 | 1458篇 |
2011年 | 1473篇 |
2010年 | 1052篇 |
2009年 | 1180篇 |
2008年 | 901篇 |
2007年 | 985篇 |
2006年 | 856篇 |
2005年 | 585篇 |
2004年 | 471篇 |
2003年 | 383篇 |
2002年 | 385篇 |
2001年 | 396篇 |
2000年 | 253篇 |
1999年 | 328篇 |
1998年 | 255篇 |
1997年 | 185篇 |
1996年 | 125篇 |
1995年 | 107篇 |
1994年 | 94篇 |
1993年 | 71篇 |
1992年 | 61篇 |
1991年 | 71篇 |
1990年 | 74篇 |
1989年 | 56篇 |
1988年 | 63篇 |
1987年 | 39篇 |
1986年 | 43篇 |
1985年 | 20篇 |
1984年 | 12篇 |
1983年 | 15篇 |
1982年 | 10篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1976年 | 4篇 |
1959年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
161.
HUANG Xiao-wen SHI Peng-fei 《高等学校化学研究》2006,22(1):73-75
IntroductionLithium ion batteries are key components of mobiletelephones and portable computers.Among the knownLi-intercalation materials for lithium ion battery cath-odes,LiCoO2,LiNiO2,and LiMn2O4have been stud-ied extensively[1—3].LiCoO2is nowused in c… 相似文献
162.
The title compound, (isopropylxanthato)(phenyl)mercury(I), was synthesized and characterized by elemental analysis, IR and thermogravimetric analysis. Density functional theory (DFT) method calculations were performed at B3LYP/CEP-121G and B3LYP/CEP-31G levels of theory, respectively. Vibrational frequencies were predicted, assigned, compared with the experimental values, and they are supported each other. The calculated results show that the strength of bond Hg—C is stronger than that of Hg—S, which is good agreement with the experimental data. The calculations of the second order optical nonlinearity and electronic absorption spectra are also performed. 相似文献
163.
微波消解火焰原子吸收光谱法连续测定金银花中铁、锌、铜和锰 总被引:8,自引:0,他引:8
采用微波消化技术,以氘灯背景校正方式,在HNO3介质中,直接用火焰原子吸收法在同一体系中连续测定了金银花中微量元素Fe、Zn、Cu、Mn,并优化了最佳实验条件.在选定条件下,检出限为Fe 0.0047 μg/mL,Zn 0.0032μg/mL,Cu 0.0052μg/mL,Mn 0.0028μg/mL,相对标准偏差为1.5%~3.2%,回收率为96.4%~103.4%.适用于金银花中微量Fe、Zn、Cu、Mn的测定. 相似文献
164.
Xi‐Zhao Wang Jiong Jia Yan Zhang Wei‐Ren Xu Wei Liu Fang‐Niu Shi Jian‐Wu Wang 《中国化学会会志》2007,54(3):643-652
The synthesis of a series of new 5‐isoxazolpenicillins is described, which were obtained by coupling substituted isoxazoles with 6‐APA. Concise large‐scale synthesis of 3,5‐disubstituted isoxazoles by 1,3‐dipolar cycloaddition using copper(I) as catalyst was also investigated. Representative compounds were assayed for antimicrobial activities, showing satisfactory antimicrobial activities against Gram‐negative bacteria. 相似文献
165.
In this study, local least squares (LLS) and principal component analysis (PCA) were applied to deal with the disturbances in a data set of chromatographic fingerprints after necessary data transformations. It has been demonstrated that PCA with standard normal variate (SNV) transformation of data led to meaningful classification of 33 different Erigeron breviscapus herbal samples. The result was also corroborated by variance squares discriminant method. The quality of herbal objects was further evaluated, and the causes of this fact have been explained from a chemical point of view. At the same time, it implied an idea for qualitative evaluation of the herbal objects with a common class pattern of chromatographic fingerprints. 相似文献
166.
A new derivative of 2,5-piperazinedione, 1,4-di(2-methoxyphenyl)-2,5-piperazinedione (I), was synthesized by the cyclocondensation reaction of N-2-methoxyphenyl chloroacetamide, and its structure was characterized by elemental analysis, IR, ^1H NMR and single crystal X-ray diffraction method. The crystal belongs to monoclinic system, space group P21/c with unit cell dimensions a=0.56934(10) nm, b=1.3880(2) nm, c=1.00329(17) nm, β= 90.376(3)°, V= 0.7928(2) nm^3, Z=2, Dc = 1.367 g·cm^-3,μ = 0.98 cm^-1, R and wR being 0.0606 and 0.1564 respectively for 1549 unique reflections with 1247 observed reflections [I〉2σ(I)]. The molecule has a crystallographically imposed symmetry center. The three rings in the molecule are each coplanar with their attached groups, excluding methyl H atoms and the H atoms attached to the piperazinedione ring, while the whole molecule is not planar, with dihedral angles of 74.7(1)° between the piperazinedione and each of the two aromatic rings. The crystal structure is stabilized by van der Waals and dipole-dipole forces. 相似文献
167.
The complexes between R3Tr (Tr = B, Al, and Ga; R = H, F, Cl, and Br) and H2X (X = O, S, and Se) were theoretically studied. The interaction energies of R3Al⋯H2X and R3Ga⋯H2X are consistent with the electronegativity of the halogen atom R (R ≠ H), but an opposite dependence is found for R3B⋯H2X. The triel bond of R3Tr⋯H2X is weaker for the heavier chalcogen donor. The dependence of triel bonding strength on the triel atom is complicated, depending on the nature of R and X. The methyl substitution of H2X causes a substantial increase in the interaction energy from −5.74 kcal/mol to −22.88 kcal/mol, and its effect is relevant to the nature of Tr, X, and R groups. For the S and Se donors, the increased percentage of interaction energy is almost the same due to the methyl substitution, which is larger than that of the O analogue. In most triel-bonded complexes, electrostatic dominates and polarization has comparable contribution. However, polarization plays a dominant role in R3B⋯ and R3B⋯ (R = Cl and Br; R′ = H and Me). 相似文献
168.
Xiu Hui Lu Hai Bin Yu Wei Rong Wu Yue Hua Xu 《International journal of quantum chemistry》2007,107(2):451-457
Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second‐order Møller–Plesset (MP2)/6‐31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero‐point energy (ZPE) and CCSD(T)/6‐31G* single‐point calculations. From the PES obtained with the CCSD(T)//MP2/6‐31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four‐membered ring intermediate, INT2, which is a barrier‐free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four‐membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier‐free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier‐free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
169.
170.
Lu Chengxue Yu Xiufang Zhang Honglin 《Journal of Thermal Analysis and Calorimetry》1997,48(2):327-329
The hydrolytic polymerization of Cr3+ at relatively high concentrations was studied by microcalorimetry. The thermal curves were determined with a 2277 thermal activity monitor. From the curves, the identified reaction heats (δrH m σ ), hydrolysis constants (K) and thermodynamic functions (δrS m σ ,δrH m σ ) were calculated at different temperatures. 相似文献