全文获取类型
收费全文 | 316篇 |
免费 | 43篇 |
国内免费 | 100篇 |
专业分类
化学 | 260篇 |
晶体学 | 11篇 |
力学 | 12篇 |
综合类 | 6篇 |
数学 | 75篇 |
物理学 | 95篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2022年 | 15篇 |
2021年 | 5篇 |
2020年 | 13篇 |
2019年 | 6篇 |
2018年 | 11篇 |
2017年 | 10篇 |
2016年 | 19篇 |
2015年 | 19篇 |
2014年 | 20篇 |
2013年 | 17篇 |
2012年 | 19篇 |
2011年 | 34篇 |
2010年 | 22篇 |
2009年 | 34篇 |
2008年 | 28篇 |
2007年 | 17篇 |
2006年 | 25篇 |
2005年 | 20篇 |
2004年 | 24篇 |
2003年 | 15篇 |
2002年 | 14篇 |
2001年 | 9篇 |
2000年 | 10篇 |
1999年 | 3篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1973年 | 1篇 |
1963年 | 1篇 |
1962年 | 1篇 |
1961年 | 2篇 |
1948年 | 1篇 |
排序方式: 共有459条查询结果,搜索用时 15 毫秒
141.
142.
利用超声方法测定北京大钟寺永乐古钟的壁厚 总被引:1,自引:0,他引:1
利用超声测厚的方法第一次测定了北京永乐古钟的壁厚。由于采用了交叉触发脉冲重叠法以及适当调节工作频率,从而解决了测量中的实际困难。测量精度约为1.5%。还根据测得大钟的直径和密度,计算出大钟连同其挂钩在内的总重量约为46.6±1吨。 相似文献
143.
The effect of energy dependence (ED) of the G matrix on RPA is investigated. Using the Hamada-Johnston potential, we calculate the 1p-1h TDA spectra of 16O. The following four cases are considered: (1) every G matrix element is approximated by its on shell value, (2) ED of G is taken intoaccount rigorously, (3) the effect of self-screening (SC) is considered, but the G matrix elements are assumed on the energy shell, (4) both SC and ED of G are taken into account. It is found that ED of G indeed causes a substantial damping effect on the backward-propagating bubbles of RPA as suggested by Wu. Further, ED of G and SC work in the same direction to diminish the difference between the energy spectra calculated by TDA and RPA. 相似文献
144.
综合人寿保险精算模型 总被引:3,自引:0,他引:3
保险是金融的重要组成部分,国际保险业发展迅速,我国保险业务较晚,资料匮乏,迫切需要引进国外先进的保险经验和保险技术,并结合我国的实际情况加以运用。本文建立了一个综合的人寿保险精算模型,其中包括生存年金,终身寿险和还本部分。通过适当的调整参数进行组合,可以获得不同的保险产品。 相似文献
145.
The title compound, [Cu3(dmpzm)4(μ-1,3-dca)2(dca)4]·2H2O (dmpzm = 1,1'-methyle- nebis(3,5-dimethyl-1H-pyrazole), dca = dicyanamide) 1·2H2O, was synthesized via the reaction of CuCl2·2H2O with dmpzm and Na(dca), and characterized by elemental analysis and IR spectra. 1·2H2O crystallizes in the monoclinic system, space group C2/m with a = 13.578(3), b = 16.769(3), c = 14.769(3)(A),β=104.10(3)°, V = 3261.4(11)(A)3, Z = 2, Dc = 1.466 g/cm3, T = 153(2) K, C56H68N34O2Cu3, Mr = 1440.09, F(000) = 1490, μ(MoKα) = 1.040 mm(1, S = 1.150, R = 0.0553 and wR = 0.1123 for 2744 observed reflections with I > 2σ(I). The central copper atom is chelated by two dmpzm ligands and coordinated by two bridging dca anions, forming a slightly distorted octahedral geometry. The two external copper atoms adopt a square-pyramidal coordination geometry, coor- dinated by one chelating dmpzm ligand and one bridging and two terminal dca anions. Two inter- molecular hydrogen bonding interactions result in the formation of a 2D (4,4) hydrogen-bonded network. 相似文献
146.
The title compound[Zn(btzb)2Cl2]·2H2O(1·2H2O,btzb=1,2-bis(5-tetrazolyl)benzene)was synthesized in situ by the[2 3] cycloaddition reaction of phthalonitrile with NaN3 in water in the presence of ZnCl2 under refluxing conditions.1·2H2O crystallizes in the monoclinic system,space group P21/c with a=9.0119(18),b=7.5566(15),c=18.076(5)(A),β=114.67(2)°,V=1118.6(4)(A)3,Z=2,Dc=1.784 g/cm3 T=223(2)K,C16H16N16O2Cl2Zn,Mr=600.74,F(000)=608,μ(MoKα)=1.393 mm-1,S=1.081,R=0.0306 and wR=0.0669 for 1896 observed reflections with Ⅰ >2σ(Ⅰ).The Zn2 ion of 1 is coordinated by four N atoms from two btzb ligands and two Cl atoms,forming a distorted octahedral coordination geometry.A number of intermolecular hydrogen bonding interactions between molecules 1 and/or the solvated water molecules result in a 3D hydrogen-bonded structure.The luminescent property of 1·2H2O was also investigated. 相似文献
147.
小分子醇对PVDF/PFSA共混膜凝胶动力学、膜结构及膜性能的影响 总被引:1,自引:1,他引:0
以小分子醇为添加剂,研究了小分子醇对聚偏氟乙烯(PVDF)/全氟磺酸(PFSA)共混超滤膜凝胶动力学、结构与渗透性能的影响.结果表明:在PVDF/PFSA铸膜液中添加乙醇时,其凝胶速率随乙醇浓度的增加而加快,超滤膜通量随乙醇浓度的增加而增大;而当添加相同浓度的4种小分子醇时,初始凝胶速率差别不大,20 s后添加乙醇的铸膜液的凝胶速率最大;添加小分子醇后超滤膜的水通量大小顺序为:异丙醇<甲醇<乙醇<正丁醇.SEM照片显示在铸膜液中添加5%乙醇所制备的共混超滤膜具有较为圆整的内腔结构与均匀的孔结构. 相似文献
148.
Fei-Chien Chang Yang Zhou Matthew Michael James Hadi M. Zareie Yoshiki Ando Jihui Yang Miqin Zhang 《Macromolecular bioscience》2023,23(1):2200389
Stem cell therapy and research for neural diseases depends on reliable reproduction of neural stem cells. Chitosan-based materials have been proposed as a substrate for culturing human neural stem cells (hNSCs) in the pursuit of clinically compatible culture conditions that are chemically defined and compliant with good manufacturing practices. The physical and biochemical properties of chitosan and chitin are strongly regulated by the degree of deacetylation (DD). However, the effect of DD on hNSC behavior has not been systematically investigated. In this study, films with DD ranging from 93% to 14% are fabricated with chitosan and chitin. Under xeno-free conditions, hNSCs proliferate preferentially on films with a higher DD, exhibiting adherent morphology and retaining multipotency. Lowering the DD leads to formation of neural stem cell spheroids due to unsteady adhesion. The neural spheroids present NSC multipotency protein expression reduction and cytoplasmic translocation. This study provides an insight into the influence of the DD on hNSCs behavior and may serve as a guideline for hNSC research using chitosan-based biomaterials. It demonstrates the capability of controlling hNSC fate by simply tailoring the DD of chitosan. 相似文献
149.
设定了Visual minteq软件F~--Al~(3+)体系的物种及平衡常数。模拟滴定和实验滴定结果的相似性证明了软件设定的合理。对F-(0.01/0.1 mol/L)-Al~(3+)(0.02 mol/L)-OH-体系的模拟和分析表明,当F与Al摩尔比相差大时,体系平衡物种分布不同,OH-掩蔽铝的机理也不同,但是在p H值11~12铝都被成功掩蔽;不同的反应机理导致F与Al摩尔比大的体系模拟和滴定曲线差别较大。进一步模拟得到了此p H值区间的最大可掩蔽铝浓度。考虑OH-对氟离子电极的影响,最终确定高氟(0.01~0.1 mol/L)高铝体系氟的测定条件:控制p H值为(11.5±0.2),不大于0.02 mol/L的铝。误差分析表明,当电势测定误差较大时,标准曲线法比标准加入法误差小。 相似文献
150.