全文获取类型
收费全文 | 6928篇 |
免费 | 1500篇 |
国内免费 | 2496篇 |
专业分类
化学 | 5482篇 |
晶体学 | 272篇 |
力学 | 341篇 |
综合类 | 280篇 |
数学 | 1052篇 |
物理学 | 3497篇 |
出版年
2024年 | 24篇 |
2023年 | 81篇 |
2022年 | 276篇 |
2021年 | 273篇 |
2020年 | 274篇 |
2019年 | 263篇 |
2018年 | 259篇 |
2017年 | 372篇 |
2016年 | 288篇 |
2015年 | 405篇 |
2014年 | 492篇 |
2013年 | 620篇 |
2012年 | 662篇 |
2011年 | 658篇 |
2010年 | 614篇 |
2009年 | 612篇 |
2008年 | 675篇 |
2007年 | 571篇 |
2006年 | 548篇 |
2005年 | 466篇 |
2004年 | 366篇 |
2003年 | 268篇 |
2002年 | 270篇 |
2001年 | 252篇 |
2000年 | 260篇 |
1999年 | 202篇 |
1998年 | 114篇 |
1997年 | 87篇 |
1996年 | 75篇 |
1995年 | 69篇 |
1994年 | 61篇 |
1993年 | 65篇 |
1992年 | 53篇 |
1991年 | 45篇 |
1990年 | 35篇 |
1989年 | 40篇 |
1988年 | 45篇 |
1987年 | 35篇 |
1986年 | 28篇 |
1985年 | 31篇 |
1984年 | 18篇 |
1983年 | 17篇 |
1982年 | 16篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1959年 | 2篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
[Zn2(PCP)(phen)(H2O)F]n 1 and {[Zn3(MCP)2(phen)2(H2O)]·2.5H2O}n 2(PCP = p-O2C(C6H4)PO33-,MCP = m-O2C(C6H4)PO33- and phen = phenanthroline) were obtained by hydrothermal synthesis and characterized by X-ray single-crystal diffraction.Compound 1 crystallizes in the monoclinic P21/c space group with a = 7.908(2),b = 20.254(3),c = 13.477(2) ,β = 107.76(3)°,V = 2055.7(8) 3,Z = 4,C20H16FN2O6PZn2,Mr = 561.10,Dc = 1.813 g/cm3,μ = 2.463 mm-1,F(000) = 1128,the final R = 0.0340 and wR = 0.0794.Compound 2 crystallizes in the monoclinic P21/n space group with a = 15.629(3),b = 18.141(4),c = 17.723(7) ,β = 121.89(2)°,V = 4267(2) 3,Z = 4,C40H31N4O13.5P2Zn3,Mr = 1041.70,Dc = 1.620 g/cm3,μ = 1.818 mm-1,F(000) = 2108,the final R = 0.0669 and wR = 0.1775.In compound 1,the tetranuclear Zn4 units are linked together by μ4-PCP3- to build 2D(4,4) layers,which are further interconnected through the μ2-bridging fluorion into a 3D framework with 1D phen ligands-filled channels.As for the 3D supramolecular framework of 2,the novel hexanuclear Zn6 units with "chair" conformation are extended by the moieties of μ4-MCP3- ligand to a 2D(4,4) layer on the bc plane,which is viewed as the 2-folded layers in 1.In both compounds,the structures are stabilized by hydrogen bonding interactions and π-π stacking interactions between the phen rings.Additionally,FT-IR spectroscopy and the fluorescent properties are discussed. 相似文献
992.
993.
Six fully optimized structures of the aza-calix[2]arene[2]-triazines/RDX supramo-lecular complexes have been obtained at the DFT-B3LYP/6-311++G** level,and the corresponding intermolecular interactions have been investigated using the B3LYP,mPWPW91 and MP2 methods at the 6-311++G** level,respectively.The natural bond orbital(NBO) and atoms in molecules(AIM) analyses have been performed to reveal the origin of interactions.To our interest,the result indicates that the strongest interaction is up to -22.34 kJ/mol after basis set superposition error(BSSE) and zero point energy(ZPE) correction at the MP2/6-311++G** level.Furthermore,the intermolecular interactions between aza-calix[2]arene[2]-triazines with the substituted amidos and RDX are stronger than those of other complexes.Thus,the complexes with amidos can be used as the candidates to increase the stability of explosive and eliminate the explosive wastewater. 相似文献
994.
To improve the activity and enantioselectivity of hyperthermophilic archaeon Aeropyrum pernix K1 esterase (APE1547) and its mutants, they were purified by acetone-treated method. It was found that the acetone treatment not only caused APE1547 and its mutants to display higher activity and enantioselectivity but also saved more than 90% of time spent in purifying them by Ni-chelating column. In hydrolysis of p-nitrophenyl caprylate, the acetone-treated APE1547 and mutant A containing the following substitutions R11G, L36P, V225A, I551L, and A564T showed 5.7- and 6.9-fold active increase, respectively. In the resolution of 2-octanol acetate, the acetone-treated mutant A had a 9-fold enantioselective increase relative to that purified by Ni-chelating column. In addition, the impact of pH, temperature, and chemical reagents on activity of APE1547 and mutant A was discussed in this paper. 相似文献
995.
Lu J Peng C Ji XJ You J Cong L Ouyang P Huang H 《Applied biochemistry and biotechnology》2011,164(7):979-990
The fermentation characteristics of Mortierella alpina were investigated in response to various nitrogen sources. Influences on nitrogen source and glucose uptake rate, mycelial
morphology of M. alpina, and pH of medium in relation to different nitrogen sources were discussed. Effects of different nitrogen sources on cell
growth, fatty acid composition, arachidonic acid (ARA), and total lipid concentration were also evaluated. It revealed that
the maximum nitrogen source uptake ratio was obtained when corn steep liquor was used as nitrogen source. When yeast extract
was used as the sole nitrogen source, glucose was completely exhausted at the end of fermentation. The maximum dry cell weight
obtained from medium with yeast extract as nitrogen source had the highest total lipid concentration. Sodium nitrate was the
favorable nitrogen source for ARA accumulation, and the highest ARA percentage in total fatty acids was obtained, 35.9%. Urea
was identified as the favorable nitrogen source for ARA production, the highest ARA concentration obtained from urea was 5.8 g/l.
Compared with inorganic nitrogen sources, organic nitrogen compounds are favorable for both cell growth and total lipids accumulation. 相似文献
996.
Qiao H Gao C Mao D Konenkov N Douglas DJ 《Rapid communications in mass spectrometry : RCM》2011,25(23):3509-3520
Methods to reduce mass shifts caused by space charge with mass‐selective axial ejection from a linear quadrupole ion trap are investigated. For axial ejection, dipole excitation is applied to excite ions at q ≈ 0.85. The trapping radiofrequency (rf) voltage is scanned to bring ions of different m/z values into resonance for excitation. In the fringing field at the quadrupole exit, excited ions gain axial kinetic energy, overcoming the trapping potential, and are ejected from the trap. Space charge causes the frequencies of ion oscillation to decrease. Thus, greater rf voltages are required to bring ions into resonance for excitation and ejection, and the ions shift to higher apparent masses in a mass spectrum. At the same time, the peaks broaden, lowering resolution. The effects of injection q value, ejection q value, excitation amplitude, quadrupole dc voltages applied to the electrodes, applying an rf voltage to the exit lens, and scan speed, on mass shifts have been studied experimentally. Most experiments were done with only ions of protonated reserpine (m/z 609.3 and its isotopic peaks) in the trap. Some experiments were done with ions of protonated reserpine and ions of m/z 622 in the trap. In general, the mass shifts are reduced with higher ejection q values, higher excitation amplitudes, with quadrupole dc applied, and at higher scan speeds. The application of quadrupole dc appears to increase the ion cloud temperature, which lowers mass shifts. Thus, a proper choice of operating conditions can reduce, but not eliminate, mass shifts caused by space charge. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
997.
998.
999.
1000.
用X射线能量色散谱(EDS)、X射线衍射(XRD)谱、拉曼光谱和电化学技术研究和比较了直接甲酸燃料电池(DFAFC)中Vulcan XC-72炭黑载Pd (Pd/XC)和大孔炭载Pd (Pd/MC)催化剂对甲酸氧化的电催化性能. 循环伏安曲线测量表明甲酸在Pd/XC和Pd/MC催化剂电极上主要氧化峰的峰电位基本相同, 在0.15 V左右, 但在Pd/MC催化剂电极上的峰电流密度比在Pd/XC催化剂上的大30%左右. 计时电流曲线测量表明, 在6000 s时, 在Pd/MC催化剂电极上的峰电流密度比在Pd/XC催化剂上的大38%左右, 这些结果说明Pd/MC催化剂对甲酸氧化的电催化活性和稳定性要好于Pd/XC催化剂. 由于Pd/MC和Pd/XC催化剂的Pd粒子平均粒径和相对结晶度相似, 因此, Pd/MC催化剂电催化性能好的原因只能归结于MC大的孔径和高的石墨化程度引起的高电导率. 相似文献