全文获取类型
收费全文 | 6336篇 |
免费 | 1166篇 |
国内免费 | 1714篇 |
专业分类
化学 | 6214篇 |
晶体学 | 323篇 |
力学 | 72篇 |
综合类 | 105篇 |
数学 | 34篇 |
物理学 | 2468篇 |
出版年
2024年 | 24篇 |
2023年 | 76篇 |
2022年 | 172篇 |
2021年 | 211篇 |
2020年 | 239篇 |
2019年 | 194篇 |
2018年 | 148篇 |
2017年 | 200篇 |
2016年 | 258篇 |
2015年 | 262篇 |
2014年 | 329篇 |
2013年 | 502篇 |
2012年 | 466篇 |
2011年 | 468篇 |
2010年 | 359篇 |
2009年 | 386篇 |
2008年 | 367篇 |
2007年 | 450篇 |
2006年 | 449篇 |
2005年 | 427篇 |
2004年 | 399篇 |
2003年 | 380篇 |
2002年 | 401篇 |
2001年 | 357篇 |
2000年 | 274篇 |
1999年 | 156篇 |
1998年 | 132篇 |
1997年 | 147篇 |
1996年 | 120篇 |
1995年 | 132篇 |
1994年 | 125篇 |
1993年 | 114篇 |
1992年 | 122篇 |
1991年 | 88篇 |
1990年 | 87篇 |
1989年 | 58篇 |
1988年 | 46篇 |
1987年 | 17篇 |
1986年 | 8篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1975年 | 2篇 |
1973年 | 6篇 |
1959年 | 2篇 |
排序方式: 共有9216条查询结果,搜索用时 15 毫秒
991.
计算机差谱法测定混合稀土元素 总被引:4,自引:1,他引:3
报道了在自装光电二极管阵列电感耦合等离子体原子发射光谱仪上采用计算机差谱法进行混合稀土元素分析。该法能有效而简便地消除光谱干扰,结果准确可靠。 相似文献
992.
根据分子动力学理论建立了液态锌铝合金ZA27的模型,结合计算机编程构造出了ZA27合金α 相与液相共存时的原子结构模型,利用递归方法计算了稀土固溶于晶粒内和富集于结晶前沿 时的电子结构.由此得出:稀土处于相界区比在晶内更稳定,从而解释了稀土在α相内溶解 度很小,结晶时富集于结晶前沿液体中的事实;稀土处于液态和晶态的结构能差相对于铝较 大解释了稀土在相界前的富集使α晶枝产生熔断、游离、增殖的观点.原子间的键级积分计 算也表明,稀土处于结晶前沿液体中与铝相比不容易结晶到晶体表面,起到阻碍晶粒长大, 细化晶粒的作用,这就从电子层次解释了稀土的变质机理.
关键词:
电子结构
液固相界原子结构模型
稀土变质机理 相似文献
993.
The synthesis of 3‐(triethoxysilyl)‐propyl isocyanate (TEPIC) modified by (3‐aminopropyl)triethoxysilane (APS) and the preparation of the corresponding organic–inorganic molecular‐based hybrid material with the two components equipped with covalent bonds is described. The coupling agent moiety is a convolution of TEPIC and APS through ? NHC(?O)NH? groups, which is applied to coordinate to RE3+ and further formed Si? O backbones after hydrolysis and polycondensation processes. For comparison and luminescence efficiency purposes, we added 2,2‐bipyridyl to the above hybrids in order to increase the conjugating effects and sensitize rare earth ions emissions. Luminescence spectra were utilized to characterize the photophysical properties of the hybrid material obtained, and the above spectroscopic data reveal that the triplet energy of 2,2‐dipyridyl in this favorable hybrid system matches with the emissive energy level of RE3+. In this way, the intramolecular energy transfer process took place within these molecular‐based hybrids and strong green and red emissions of RE3+ have been achieved. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
994.
Elzain M. Al Rawas A. Yousif A. Gismelseed A. Rais A. Al-Omari I. Bouziane K. Widatallah H. 《Hyperfine Interactions》2004,158(1-4):205-209
Time differential perturbed γ-γ angular correlation technique was used to measure the magnetic hyperfine field (MHF) at Tb
sites in the intermetallic compound Tb3In5 using the 140La → 140Ce nuclear probe. The measurements were carried out in the temperature range of 8 to 295 K. Two different temperature dependent
magnetic frequencies were observed below 30 K, which were assigned as 140Ce substituting the two inequivalent Tb sites in the orthorhombic structure of Tb3In5. The temperature dependence of MHF also shows a possible deviation from an expected Brillouin-like behavior for temperatures
below 18 K. A Néel transition at 27 K was observed from magnetization measurements in the samples. The magnetization as a
function of the applied magnetic field was measured at two temperatures, 5 and 40 K, and the results show antiferromagnetic
and a typical paramagnetic behavior, respectively. In both cases it was not observed saturation under high magnetic field. 相似文献
995.
Q. Zhang G.H. Rao H.Z. Dong Y.G. Xiao X.M. Feng G.Y. Liu Y. Zhang J.K. Liang 《Physica B: Condensed Matter》2005,370(1-4):228-235
The crystal structure and magnetic properties of a series of ordered double perovskite oxides (Sr2−3xLa2xBax)FeMoO6 (0x0.3) have been investigated. X-ray powder diffraction reveals that the crystal structure of the compounds changes from a tetragonal I4/m lattice to a cubic lattice around x=0.2. Though the nominal average size of the A site cation of (Sr2−3xLa2xBax)FeMoO6 is designed to be almost independent of x, the refinements of the crystal structure show that the lattice constants increase with x in both the tetragonal and the cubic phase regions due to electron doping. As the x increases, the degree of cationic ordering on the B site is decreased pronouncedly, while the Curie temperature of the compounds is nearly unchanged. The saturation magnetization of the compounds decreases with x and shows a linear dependence on the degree of cation ordering. The resistivity of the parent compound shows a semiconducting behavior below room temperature, but those of the doped samples exhibit a metal–semiconductor transition. A correlation between the resistivity and metal-semiconducting transition temperature (TM−S) is observed. The resistivity and TM−S of the compounds decrease with x for x0.2 and increase for x0.2. Magnetoresistance of the compounds is reduced by the La/Ba doping. All these observations can be understood based on the interplay of the electron doping, change in bandwidth and the anti-site defect concentration. 相似文献
996.
997.
Rare-earth doped fibers suffer from relatively high attenuation in comparison to conventional communication fibers. In order to improve the properties of such fibers, understanding of the mechanisms involved in scattering effects is of great importance. The effect of Nd-doping, Al-codoping and of the drawing temperature on the scattering effect was investigated for several different optical fibers. To this end a measuring setup was realized which allows absolute measurements of the scattering coefficient at different positions along a fiber. Also separation of total attenuation and scattering effects is possible. The presented results indicate scattering effects as primary source for increased attenuation compared with undoped fibers. 相似文献
998.
L. Seetha Lakshmi V Sridharan DV Natarajan V Sankara Sastry T S Radhakrishnan 《Pramana》2002,58(5-6):1019-1026
Mn site is substituted with closed shell ions (Al, Ga, Ti, Zr and a certain combination of Zr and Al) and also with Fe and
Ru ions carrying the magnetic moment (S=5/2 and 2 respectively) at a fixed concentration of 5 at %. Substitution did not change either the crystal symmetry or the
oxygen stoichiometry. All substituents were found to suppress both the metal-insulator and ferromagnetic transition temperatures
(T
p(ρ) and T
C, respectively) to varied extents. Two main contributions identified for the suppression are the lattice disorder arising
due to difference in the ionic radii between the substituent (r
M) and the Mn3+ ion (r
Mn
3+) and in the case of the substituents carrying a magnetic moment, the type of magnetic coupling between the substituent and
that of the neighboring Mn ion. 相似文献
999.
苯甲酸及其一氯代衍生物与邻菲罗啉或联吡啶的稀土铕三元配合物的荧光性质 总被引:16,自引:4,他引:12
本文报道合成了八种苯甲酸及其一氯代衍生物与邻菲罗啉或联吡啶的稀土铕三元配合物并测得了荧光光谱。通过改变第一和第二配体,研究了配合物发光峰强度、位置和分裂间隔等的变化情况,依据荧光发射峰的分裂推断出Eu~(3 )离子所处的局域环境对称性大致都是C_1,C_2或C_s。虽然配合物的局域对称性大致相同,但由于配体场作用和能级匹配等原因,发光峰位置和强度是不同的;讨论了第一配体分子结构的变化对荧光性能的影响,认为分子自身结构的对称性可能对提高配合物发光性能有较大影响;实验还观察到第二配体也参与配位且对发光性能影响较大。同时看到发光峰的分裂间隔大小变化和发光峰的相对强度的变化是一致的。 相似文献
1000.