全文获取类型
收费全文 | 995篇 |
免费 | 27篇 |
国内免费 | 3篇 |
专业分类
化学 | 524篇 |
晶体学 | 26篇 |
力学 | 37篇 |
数学 | 93篇 |
物理学 | 345篇 |
出版年
2022年 | 7篇 |
2021年 | 12篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 11篇 |
2017年 | 14篇 |
2016年 | 32篇 |
2015年 | 12篇 |
2014年 | 18篇 |
2013年 | 54篇 |
2012年 | 44篇 |
2011年 | 57篇 |
2010年 | 41篇 |
2009年 | 33篇 |
2008年 | 59篇 |
2007年 | 43篇 |
2006年 | 52篇 |
2005年 | 40篇 |
2004年 | 27篇 |
2003年 | 24篇 |
2002年 | 24篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 28篇 |
1992年 | 24篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1964年 | 5篇 |
1955年 | 5篇 |
排序方式: 共有1025条查询结果,搜索用时 312 毫秒
101.
Amitava Datta 《Pramana》1993,40(6):L503-L509
It is argued with the help of an illustrative model, that the inter species hierarchy among the fermion masses and the quark
mixing angles can be accommodated naturally in the standard model with (approximate) flavor democracy provided there are three
exactly massless neutrinos and four families of sequential quark-leptons with all members of the fourth family having roughly
equal masses. The special problem of light neutrino masses (if any) and possible solutions are also discussed. 相似文献
102.
A temperature-dependent integrated kinetics for the overall process of photosynthesis in green plants is discussed. The C4 plants are chosen and in these plants, the rate of photosynthesis does not depend on the partial pressure of O2. Using some basic concepts like chemical equilibrium or steady state approximation, a simplified scheme is developed for
both light and dark reactions. The light reaction rate per reaction center (R′
1) in thylakoid membrane is related to the rate of exciton transfer between chlorophyll neighbours and an expression is formulated
for the light reaction rate R′
1. A relation between R′
1 and the NADPH formation rate is established. The relation takes care of the survival probability of the membrane. The CO2 saturation probability in bundle sheath is also taken into consideration. The photochemical efficiency (ϕ) is expressed in terms of these probabilities. The rate of glucose production is given by R
glucose = (8/3)(R′
1
v
L
)ϕ(T)
g
(T) ([G3P]/[P
i]2
leaf)SS
Q
G3P→glucose where g is the activity quotient of the involved enzymes, and G3P represent glycealdehyde-3-phosphate in steady state. A Gaussian
distribution for temperature-dependence and a sigmoid function for de-activation are incorporated through the quotient g. In general, the probabilities are given by sigmoid curves. The corresponding parameters can be easily determined. The theoretically
determined temperature-dependence of photochemical efficiency and glucose production rate agree well with the experimental
ones, thereby validating the formalism. 相似文献
103.
104.
Topochemical Transformations of CaX2 (X=C,Si, Ge) to Form Free‐Standing Two‐Dimensional Materials 下载免费PDF全文
Saied Md. Pratik A. Nijamudheen Prof. Dr. Ayan Datta 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(50):18454-18460
Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high‐yield synthesis of pristine, defect‐free two‐dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid‐state dispersion‐corrected calculations, mechanisms for such transformations are elucidated that provide an in‐depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans‐bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans‐bent structure of acetylene in its first excited state (S1) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase‐h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane. 相似文献
105.
106.
107.
The reactions of Vaska’s complex [IrCl(CO)(PPh3)2] with 2-(coumaryl-6-azo)imidazole (CZ-H) and its derivatives (CZ-X) have synthesized [Ir(CZ)(CO)(PPh3)2] and [Ir(CZ-X)(CO)(PPh3)2]. All the complexes have been characterized by FT-IR, UV-Vis, 1H NMR and FAB-MS spectroscopy. The structural confirmation has been done in one case, by a single crystal X-ray diffraction study, which shows a distorted square pyramidal geometry around the central Ir atom. The complexes are emissive at room temperature. The cyclic voltammetry of the complexes shows a metal centered irreversible oxidation and ligand centered quasireversible reduction couples. To get an insight into the electronic structure, absorption spectra and electrochemical properties, detailed calculations on all three complexes have been performed at the DFT level. 相似文献
108.
In this paper, a permutation-based genetic algorithm (GA) is applied to the NP-hard problem of arranging a number of facilities on a line with minimum cost, known as the single row facility layout problem (SRFLP). The GA individuals are obtained by using some rule-based as well as random permutations of the facilities, which are then improved towards the optimum by means of specially designed crossover and mutation operators. Such schemes led the GA to handle the SRFLP as an unconstrained optimization problem. In the computational experiments carried out with large-size instances of sizes from 60 to 80, available in the literature, the proposed GA improved several previously known best solutions. 相似文献
109.
110.
Structures and electronic properties of clusters of an all-Si analogue of graphene, silicene, have been studied through quantum chemical calculations. The structures of the six-membered rings show interesting chair like puckering, which for large sheet-like clusters form ordered ripples. Binding energies, HOMO-LUMO gaps and polarizabilities for the silicene clusters show interesting monotonic trends analogous to polyacenes. Stacking of two silicene layers leads to the formation of closed 3D clusters with high symmetry and strong Si-Si bonds. The heat of hydrogenation of silicene to form silicanes is overwhelmingly exothermic and leads to the opening up of the HOMO-LUMO gaps. Thus, analogous to graphanes, silicanes are predicted to be interesting materials for hydrogen storage and for their band engineering properties. 相似文献