全文获取类型
收费全文 | 598篇 |
免费 | 35篇 |
国内免费 | 4篇 |
专业分类
化学 | 452篇 |
晶体学 | 13篇 |
力学 | 17篇 |
数学 | 58篇 |
物理学 | 97篇 |
出版年
2024年 | 3篇 |
2023年 | 1篇 |
2022年 | 29篇 |
2021年 | 29篇 |
2020年 | 20篇 |
2019年 | 18篇 |
2018年 | 25篇 |
2017年 | 20篇 |
2016年 | 18篇 |
2015年 | 24篇 |
2014年 | 29篇 |
2013年 | 39篇 |
2012年 | 20篇 |
2011年 | 54篇 |
2010年 | 23篇 |
2009年 | 26篇 |
2008年 | 23篇 |
2007年 | 33篇 |
2006年 | 20篇 |
2005年 | 30篇 |
2004年 | 16篇 |
2003年 | 22篇 |
2002年 | 19篇 |
2001年 | 12篇 |
2000年 | 10篇 |
1999年 | 3篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 6篇 |
1994年 | 2篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1984年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1938年 | 1篇 |
排序方式: 共有637条查询结果,搜索用时 15 毫秒
551.
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
552.
Muhammad Athar Abbasi Muhammad Arif Lodhi Viqar Uddin Ahmad Aziz-ur-Rehman Naseem Shahzad Muhammad Iqbal Choudhary 《Chemistry of Natural Compounds》2011,47(5):693-696
The mechanism of inhibition of α-chymotrypsin enzyme by two flavone glycosides having an apigenin basic skeleton, namely, apigenin 7-O-β-D(-6′′-p-E-coumaroyl)-glucoside (1) and apigenin 7-O-β-D-glucoside (2), isolated from Clematis orientalis L. (Ranunculaceae), was investigated. Lineweaver-Burk and Dixon plots and their secondary
replots have shown that both of the compounds are competitive inhibitors of this enzyme. Ki values for 1 and 2 were found to be 71.75 ± 0.85 and 680.11 ± 0.11 μM, respectively. 相似文献
553.
Muhammad Imran Khan Zafar Iqbal Lateef Ahmad Yasar Shah Shabnam Nazir Abbas Khan Fazli Nasir 《Chromatographia》2011,73(9-10):929-939
A highly sensitive, precise, and accurate reversed-phase high-performance liquid-chromatography/electrochemical detection method for simultaneous determination of the endogenous free ??-lipoic acid and dihydrolipoic acid in biological matrices was developed and validated. The two analytes were extracted from the samples with acetonitrile/10% metaphosphoric acid solution(aqueous) (50/50 v/v). To determine the total lipoic acid, samples were treated with tris(2-carboxyethyl)phosphine solution in phosphate buffer, pH 2.5 with 85% orthophosphoric acid prior to deproteination. The two analytes were separated on a C18 (150 × 4.6 mm, 5 ??m) analytical column using acetonitrile-50 mM phosphate buffer, pH 2.5 with 85% orthophosphoric acid (35/65 v/v) as the isocratic mobile phase pumped at a flow rate of 2.0 mL min?1 at the column oven temperature of 35 °C. The column eluents were monitored at a potential of 0.9 V. These analytes were efficiently resolved in <7 min. The present method was sufficiently robust and specific for simultaneous determination of the two analytes and demonstrated acceptable values for linearity (r 2 = 0.999 in the range of 0.1?C500 and 0.25?C1,000 ng mL?1 for ??-lipoic acid and dihydrolipoic acid, respectively), recovery (>97%), precision (RSD% <2), and sensitivity (on column limit of detection, 150 and 375 fg for ??-lipoic acid and dihydrolipoic acid, respectively and limit of quantification: 0.5 and 1.25 pg for ??-lipoic acid and dihydrolipoic acid, respectively), indicating that the proposed method was more sensitive, precise, economical, and versatile, and has higher throughput than the previously reported methods for simultaneous determination of the two analytes. 相似文献
554.
Abstract
The previously reported [Ru(naph)4]2+ complex (naph = 1,8-naphthyridine) has been prepared by a simplified route using [RuCl2(1,5-COD)] x (COD = cyclooctadiene) as starting material and isolated as its tetraphenylborate salt. The salt crystallizes in the monoclinic space group P21/n with a = 13.6531(3) ?, b = 12.5389(4) ?, c = 20.0349(5) ?, β = 96.5884(15)o, V = 3407.22(16) ?, D calc = 1.300 at 150(1) K. The dication has crystallographically imposed inversion symmetry. Although the iron analogue has been found to have a coordination number of eight, the ruthenium complex is only six-coordinate, which is achieved by the presence of two monodentate and two bidentate 1,8-naphthyridine ligands. The observation of a higher coordination number for Fe(II) vs. Ru(II) can be explained by the high spin nature of the iron complex. A byproduct complex, [Ru(1,5-COD)(naph)2][B(C6H5)4]2, could also be synthesized, isolated pure, and structurally characterized. The organometallic complex possesses an 18 electron configuration by virtue of the dicationic metal center being coordinated by the diene ligand and all four nitrogen lone pairs. This salt crystallizes in the triclinic space group \textP[`1] {\text{P}}\bar{1} with a = 12.9538(3) ?, b = 14.9485(3) ?, c = 17.4291(3) ?, α = 69.0649(11)o, β = 78.3211(9)o, γ = 78.5629(10)o, V = 3057.50(11) ?3, D calc = 1.293 at 150(1) K. 相似文献555.
Arif Ankarali Mete Kalyoncu Fatih M. Botsali Turan şişman 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2013,19(3-4):187-200
In this study, the equation of motion of a single link flexible robotic arm with end mass, which is driven by a flexible shaft, is obtained by using Hamilton's principle. The physical system is considered as a continuous system. As a first step, the kinetic energy and the potential energy terms and the term for work done by the nonconservative forces are established. Applying Hamilton's principle the variations are calculated and the time integral is constructed. After a series of mathematical manipulations the coupled equations of motion of the physical system and the related boundary conditions are obtained. Numerical solutions of equations of motion are obtained and discussed for verification of the model used. 相似文献
556.
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
557.
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
558.
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559.
560.