全文获取类型
收费全文 | 558篇 |
免费 | 27篇 |
国内免费 | 2篇 |
专业分类
化学 | 386篇 |
晶体学 | 12篇 |
力学 | 16篇 |
数学 | 55篇 |
物理学 | 118篇 |
出版年
2024年 | 2篇 |
2022年 | 21篇 |
2021年 | 25篇 |
2020年 | 14篇 |
2019年 | 14篇 |
2018年 | 18篇 |
2017年 | 21篇 |
2016年 | 18篇 |
2015年 | 18篇 |
2014年 | 22篇 |
2013年 | 38篇 |
2012年 | 22篇 |
2011年 | 47篇 |
2010年 | 24篇 |
2009年 | 22篇 |
2008年 | 23篇 |
2007年 | 36篇 |
2006年 | 24篇 |
2005年 | 36篇 |
2004年 | 16篇 |
2003年 | 23篇 |
2002年 | 16篇 |
2001年 | 12篇 |
2000年 | 10篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1975年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有587条查询结果,搜索用时 15 毫秒
571.
We have developed a new class of circular distributions named wrapped weighted exponential distributions. The estimation of unknown parameters along with some characteristics of these distributions is also investigated. Some theorems that relate the distribution to some other circular distributions are established and we clarify their modeling potential using a classical data set on movements of sea stars. 相似文献
572.
Rehan Basta Atta M. Arif Richard D. Ernst 《Journal of chemical crystallography》2011,41(10):1433-1437
Abstract
The reactions of Ti(C5H5)(6,6-dmch)(PMe3) and Zr(C5H5)(6,6-dmch)(PMe3)2 with two equivalents of C6H5C(H)=N(C6H5) have led to couplings of the imines to the 1,5 (terminal) positions of the 6,6-dmch ligands. In both cases the PMe3 ligands are lost, and the metal complexes each attain 18 electron configurations via η5-(C5H5), η3-allyl, and bis(π-amide) coordination. The complexes each crystallize in the monoclinic space group P21/c. For the titanium complex, a = 10.4336(1) ?, b = 9.0407(1) ?, c = 32.5547(6) ?, β = 94.2939(5)°, V = 3062.17(7) ?3, Dcalc = 1.264 at 200(1) K. For the zirconium complex, a = 13.8900(3) ?, b = 10.0873(2) ?, c = 22.4543(6) ?, β = 90.7638(7)°, V = 3145.85(12) ?3, Dcalc = 1.322 at 150(1) K. 相似文献573.
574.
Polynkin P Polynkin A Panasenko D Peyghambarian N Mansuripur M Moloney J 《Optics letters》2006,31(5):592-594
We report on a passively mode-locked all-fiber laser oscillator at 1.5 microm based on heavily doped phosphate-glass active fiber. An active fiber only 20 cm long is sufficient to produce as much as 2.4 W of average output power directly from the oscillator. The width of the mode-locked pulses varies from 8 ps at the lowest output power in the mode-locked state to 44 ps at the highest power. Our picosecond laser oscillator features a high repetition rate of 95 MHz and high peak pulse power of approximately 540 W. The oscillator combines the convenience of all-fiber construction with power performance that was previously achievable only with mode-locked bulk-optic laser oscillators or more complex systems involving fiber amplifiers. 相似文献
575.
We propose and demonstrate an active Q-switched fiber laser using a high-Q microsphere resonator as the Q-switching element. The laser cavity consists of an Er-doped fiber as the gain medium, a glass microsphere reflector (coupled through a fiber taper) at one end of the cavity, and a fiber Bragg grating reflector at the other end. The reflectivity of the microsphere is modulated by changing the gap between the microsphere and the fiber taper. Active Q switching is realized by oscillating the microsphere in and out of contact with the taper. Using this novel technique, we have obtained giant pulses (maximum peak power approximately 102 W, duration approximately 160 ns) at a low pump-power threshold (approximately 3 mW). 相似文献
576.
Khan KM Saify ZS Arif Lodhi M Butt N Perveen S Maharvi GM Choudhary MI Atta-Ur-Rahman 《Natural product research》2006,20(6):523-530
A variety of piperidines (2-12, 14-26) with variable substituents at N-atoms have been synthesized and evaluated as urease inhibitors. The synthesized compounds showed varying degree of urease inhibitory activity ranging from 31.97 to 254 microM. The size and electron-donating or -withdrawing effects of substituents influence the activity, which lead to the formation of urease inhibitors. 相似文献
577.
The solution structural and formyl substrate reactivity properties of a nitrogen/sulfur-ligated zinc hydroxide complex, [(bmnpaZn)2(mu-OH)2](ClO4)2 (1, bmnpa = N,N-bis-2-(methylthio)ethyl-N-((6-neopentylamino-2-pyridyl)methyl)amine), in acetonitrile and methanol are reported. In CH3CN, 1 has a binuclear cation [(bmnpaZn)2(mu-OH)2]2+ that is stabilized by secondary hydrogen bonding and CH/pi interactions involving the bmnpa chelate ligand. In CH3OH, 1 undergoes reaction with solvent to yield a zinc methoxide species, as determined by 1H NMR and electrospray mass spectral analysis. Treatment of 1 with methyl formate in CH3CN results in stoichiometric hydrolysis of the formyl ester to produce [(bmnpa)Zn(O2CH)]ClO4 (2) and methanol. The formate complex was identified via independent synthesis and characterization (X-ray crystallography, 1H and 13C NMR, FTIR, LRFAB-MS, conductance, and elemental analysis). In the solid state, 2n has a formate-bridged coordination polymer-type structure. However, in CH3CN, 2 behaves as 1:1 electrolyte, indicating cleavage of the polymer structure into mononuclear [(bmnpa)Zn(O2CH)]ClO4 species. Treatment of 1 with a stoichiometric amount of formanilide in CH3CN for 48 h at 45 degrees C results in decomposition of the zinc hydroxide complex to yield the free bmnpa ligand and an inorganic solid, presumably a zinc hydroxide or oxide species. Treatment of 1 with a stoichiometric amount of ethyl formate in CD3OD results in rapid, quantitative transesterification of the formyl carboxylate ester. A control reaction indicates that this transesterification reaction does not occur on the same time scale in the absence of the catalyst. Treatment of 1 with an excess of ethyl formate in CD3OD results in catalytic formyl carboxylate ester transesterification, with approximately 1000 turnovers in 60 min at 22(1) degrees C. Treatment of a CD3OD solution of 1 (0.5 equiv) with formanilide (1 equiv) results in the formation of aniline, d3-methyl formate, and the zinc formate complex 2. While aniline is produced stoichiometrically, the yield of d3-methyl formate varied from 30 to 50%, and the yield of 2 varied from 50 to 70% in repetitive experiments. Formation of both d3-methyl formate and 2 indicates that both methanolysis and hydrolysis reactions take place. 相似文献
578.
The reaction of V(III)(THF)3Cl3 with NEt(4)CN in acetonitrile (MeCN) forms (NEt4)3[V(III)(CN)6].4MeCN (1), which after characterization was used as a molecular building block toward the synthesis of Prussian blue structured magnets. The reaction of 1 with [Cr(II)(NCMe)4](BF4)2 forms Cr(II)(0.5)Cr(III)[V(II)(CN)6].zMeCN via internal electron transfer, whose structure and magnetic properties are dependent on the degree of solvation, z. When solvated, Cr(II)(0.5)Cr(III)[V(II)(CN)6].1.2MeCN (2) is a mixture of crystalline and amorphous fractions that yield a material with two magnetic phases: bulk ferrimagnetic phase/crystalline [faced-centered-cubic lattice with a = 10.55(2) A] and cluster-glass phase/amorphous. The bulk ferrimagnetic phase exhibits a critical temperature, Tc, of 110 K, while the amorphous cluster-glass phase exhibits a freezing temperature, Tf, of approximately 25 K. Amorphous Cr(II)(0.5)Cr(III)[V(II)(CN)6].0.1MeCN (3) was determined to be the pure cluster-glass phase. This is an overall enhancement of 85 K (350%) in the magnetic ordering temperature via solvation, z. The coercivity was also increased 4-fold from 890 (2) and 3900 Oe (3) via desolvation. 相似文献
579.
Tetranuclear [Cu(II)4(OH)4(aib)4)](hfac)4 (1; aib = 2-methyl-2-amino-4-iminopentane; hfac = hexafluoroacetylacetonate) forms from the reaction of aqueous ammonia and Cu(hfac)2.2H2O in acetone. The structure of 1 reveals that four noncoordinating hfac- counterions stabilize the distorted cubane complex via multiple H-bonding contacts. Magnetic susceptibility studies reveal that cubane-like 1 is best described as a pair of independent antiferromagnetically coupled dimers with g = 2.10 and J/kB = -298 K (207 cm(-1)) (H = -2JS1.S2). 相似文献
580.
S. K. Durrani A. H. Qureshi S. Qayyum M. Arif 《Journal of Thermal Analysis and Calorimetry》2009,95(1):87-91
The effects of barium substitution for Bi or Sr sites on the growth of superconducting phases have been studied. The sol spray
process has been used to synthesis the Bi-Sr-Ca-Cu-O (BSCCO) and Ba-BSCCO homogeneous ceramic powders. Thermogravimetric (TG),
differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques were employed
to characterize the synthesized materials. The electrical resistance was measured by classical d.c. fourprobe technique. It
has been observed that sol spray process has affected the physico-chemical properties of the materials and also avoid the
use of chelating agent as in the case of sol gel process. In addition to the Bi2Sr2CaCu2O8 (2212) phase Ba doped specimens also contained Bi2Sr2Ca2Cu3O10 (2223), BaBiO3, BaCuO2 and CuO phases.
The results revealed that the specific effect of barium-doping on either sites (Bi or Sr) seems to avoid the formation of
higher volume fraction of the low T
c phase and promoting the formation of BiBaO3, BaCuO2 and CuO along with formation of a high T
c 2223 phase. The substitution of Ba on either sites (Bi or Sr) lower the sintering temperature for the formation of high T
c (small volume fraction) however, the Ba doped specimens also contained non-superconducting phases. 相似文献