全文获取类型
收费全文 | 969篇 |
免费 | 34篇 |
国内免费 | 2篇 |
专业分类
化学 | 653篇 |
晶体学 | 22篇 |
力学 | 15篇 |
数学 | 127篇 |
物理学 | 188篇 |
出版年
2023年 | 15篇 |
2022年 | 20篇 |
2021年 | 15篇 |
2020年 | 37篇 |
2019年 | 30篇 |
2018年 | 28篇 |
2017年 | 16篇 |
2016年 | 29篇 |
2015年 | 20篇 |
2014年 | 28篇 |
2013年 | 57篇 |
2012年 | 52篇 |
2011年 | 78篇 |
2010年 | 35篇 |
2009年 | 28篇 |
2008年 | 51篇 |
2007年 | 56篇 |
2006年 | 36篇 |
2005年 | 43篇 |
2004年 | 26篇 |
2003年 | 24篇 |
2002年 | 24篇 |
2001年 | 13篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1991年 | 7篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1971年 | 6篇 |
1970年 | 6篇 |
1969年 | 8篇 |
1968年 | 8篇 |
1964年 | 4篇 |
1963年 | 6篇 |
1962年 | 8篇 |
1961年 | 11篇 |
1958年 | 7篇 |
1955年 | 5篇 |
排序方式: 共有1005条查询结果,搜索用时 15 毫秒
101.
B. Aronov A. R. Davis T. K. Dey S. P. Pal D. C. Prasad 《Discrete and Computational Geometry》1998,19(4):553-574
We extend the concept of the polygon visible from a source point S in a simple polygon by considering visibility with two types of reflection, specular and diffuse. In specular reflection a light ray reflects from an edge of the polygon according to the rule: the angle of incidence equals
the angle of reflection. In diffuse reflection a light ray reflects from an edge of the polygon in all inward directions.
Several geometric and combinatorial properties of visibility polygons under these two types of reflection are described, when
at most one reflection is permitted. We show that the visibility polygon Vs(S) under specular reflection may be nonsimple, while the visibility polygon Vd(S) under diffuse reflection is always simple. We present a Θ(n
2
) worst-case bound on the combinatorial complexity of both Vs(S) and Vd(S) and describe simple O(n
2
log
2
n) time algorithms for constructing the sets.
Received September 27, 1995, and in revised form October 24, 1997. 相似文献
102.
Supratim Biswas Rajib Dey Siddhartha Mukherjee Pataki C. Banerjee 《Applied biochemistry and biotechnology》2013,170(7):1547-1559
Extraction of metals (Ni, Co) from chromite overburden of Sukinda mines of Orissa, India, with the culture filtrate of Aspergillus niger was studied. Results showed that the amounts of metals leached varied directly with reaction temperature and period of fermentation. The culture filtrate was analyzed for citric and oxalic acids, and contained only oxalic acid—the concentration of which increased with time. Although this acid played the major role in leaching of metals, other unidentified metabolites present in the culture filtrate influenced the dissolution of the metals significantly. Maximum recovery of metals from raw and roasted ore samples was achieved at 80 °C with the 21-day culture filtrate containing the highest amount of oxalic acid. Under identical experimental conditions, much higher amounts of the metals were leached from roasted ore. Microstructures of the ore particles were studied by scanning electron microscopy and transmission electron microscopy; the bonding behaviors of metal compounds were identified by Fourier transform infrared spectroscopy which showed that the metals were leached after chelation with oxalic acid. 相似文献
103.
We employ a layer-by-layer adsorption technique for deposition on solid substrates of polyionic films of Chicago Sky Blue. Film growth was significant with the increasing number of layers. Photochemical properties of these films are investigated for different ionic concentrations. A significant blue shift is observed with the increasing ionic strength of the solution, suggesting the formation of aggregation. Temperature effect studies show some preferable reorientation of molecules in the film during cooling process. Most remarkable observation is that the absorption intensity is the highest for a particular degree of inclination. Deposition time is fixed at 15rain because adsorption kinetics results show saturation after 15 min. 相似文献
104.
Detailed structural analysis of the X-Ray diffraction pattern of La0.9Gd0.1FeO3 (LGFO) synthesized via the solid state reaction reveals distorted perovskite structure. Magnetization studies show a characteristic of weak ferromagnetism which is believed to be induced by canted anti-ferromagnetically coupled spins. When explored in a wide temperature and frequency range, AC electrical properties reveal interesting relaxation mechanisms. LGFO exhibits relatively high dielectric permittivity (ε′~4×103) close to room temperature. Temperature dependent magneto-dielectric response appears (magnetodielectric ~−5.5%) even at low magnetic field ~5 kOe. Attractive electrical, magnetic and magnetoelectric properties of the present LGFO at relatively higher temperature makes it a very promising material for application in devices. 相似文献
105.
This article deals with isomeric ruthenium complexes [RuIII(LR)2(acac)] (S=1/2) involving unsymmetric β‐ketoiminates (AcNac) (LR=R‐AcNac, R=H ( 1 ), Cl ( 2 ), OMe ( 3 ); acac=acetylacetonate) [R=para‐substituents (H, Cl, OMe) of N‐bearing aryl group]. The isomeric identities of the complexes, cct (cis‐cis‐trans, blue, a ), ctc (cis‐trans‐cis, green, b ) and ccc (cis‐cis‐cis, pink, c ) with respect to oxygen (acac), oxygen (L) and nitrogen (L) donors, respectively, were authenticated by their single‐crystal X‐ray structures and spectroscopic/electrochemical features. One‐electron reversible oxidation and reduction processes of 1 – 3 led to the electronic formulations of [RuIII(L)(L ? )(acac)]+ and [RuII(L)2(acac)]? for 1 +‐ 3 + (S=1) and 1? – 3? (S=0), respectively. The triplet state of 1 +‐ 3 + was corroborated by its forbidden weak half‐field signal near g≈4.0 at 4 K, revealing the non‐innocent feature of L. Interestingly, among the three isomeric forms ( a – c in 1 – 3 ), the ctc ( b in 2 b or 3 b ) isomer selectively underwent oxidative functionalization at the central β‐carbon (C?H→C=O) of one of the L ligands in air, leading to the formation of diamagnetic [RuII(L)(L ′ )(acac)] (L ′ =diketoimine) in 4 / 4′ . Mechanistic aspects of the oxygenation process of AcNac in 2 b were also explored via kinetic and theoretical studies. 相似文献
106.
Reactions of OH radicals and some one-electron oxidants with 2-aminopyridine (2-AmPy) and 3-aminopyridine (3-AmPy) were studied in aqueous solutions using pulse radiolysis technique. The OH adduct of 2-AmPy at pH 9 has an absorption maximum at 360 nm along with a weak absorption band in the visible region and was found to be reactive with oxygen. The rate constant for its reaction with O2 was determined to be 1.0×108 dm3 mol−1 s−1. At pH 4 also, the OH adduct of 2-AmPy has an absorption band at 360 nm. However, there are differences in the absorption at other wavelengths. From the plot of ΔOD vs. pH at 340 nm, the pKa of the OH adduct was determined to be 6.5. Among the specific oxidants, only SO4−√ radicals were able to oxidize 2-AmPy. In the case of 3-aminopyridine (3-AmPy), the transient species formed by OH radical reaction at pH 9 has an absorption maximum at 410 nm with shoulder bands on both the sides. Its absorption spectrum at pH 4 was different indicating the existence of a pK value for the OH adduct. pKa of 3-AmPy-OH radical adduct species was evaluated to be 5.7. This adduct species was also found to be reactive with oxygen (k=7.6×106 dm3 mol−1 s−1). Specific one-electron oxidants like N3√, Br2−√ C2−√ and SO4−√ were able to oxidize 3-AmPy indicating that it is easier to oxidize 3-AmPy as compared to 2-AmPy. 相似文献
107.
Bijoy K. Dey 《Journal of mathematical chemistry》2011,49(9):2032-2052
The optimal creation of a reduced space that effectively captures the long timescale dynamics of a non-linear molecular system
over a range of frequencies is described. The technique builds on a previously developed subspace method based on linear constant
projective transformation of the original full space. The present work attempts to propose transformation that are spatially
dependent thereby leading to an effective subspace for better representing the dynamics of interests. The algorithm seeks
out an optimal transformation consistent with desired low frequency motion in a rather general way. The method is demonstrated
for a six-dimensional nonlinear system reduced to two-dimensions. Superior performance is found in evaluating ensemble-averaged
classical dynamical properties. 相似文献
108.
S.?Dey S.S.?Dasgupta C.C.?Dey P.?Bhattacharya 《The European Physical Journal A - Hadrons and Nuclei》2003,16(2):193-197
A kinematically complete experiment has been performed to study the α-n final-state interaction (FSI) in the α + d ↦α + p
+ n break-up reaction at 50 MeV incident energy for the alpha-particles. For this, we have chosen four pairs of correlation
angles for the outgoing alpha and protons. These are ( θα = 18°, θp = 42°), ( θα = 20°, θp = 45°), ( θα = 22°, θp = 42°) and ( θα = 22°, θp = 47°), selected kinematically where the allowed phase spaces are in favor of the α-n final-state interaction. Our experimental
data show strong α-n FSI in all the selected configurations. Also, the FSI is found to be stronger at the lower alpha-particle
energy when two FSI peaks appear in the same configuration.
Received: 17 June 2002 / Accepted: 3 October 2002 / Published online: 4 February 2003
RID="a"
ID="a"e-mail: dey_s2001@yahoo.com; Present address: 445 Waupelani Drive, Apt. F3, State College, PA 16801, USA.
Communicated by M. Gar?on 相似文献
109.
Kaushik Dey Shebeeb Kunjattu H. Anurag M. Chahande Rahul Banerjee 《Angewandte Chemie (International ed. in English)》2020,59(3):1161-1165
Covalent organic frameworks (COFs) have attracted attention due to their ordered pores leading to important industrial applications like storage and separation. Combined with their modular synthesis and pore engineering, COFs could become ideal candidates for nanoseparations. However, the fabrication of these microcrystalline powders as continuous, crack‐free, robust films remains a challenge. Herein, we report a simple, slow annealing strategy to construct centimeter‐scale COF films ( Tp‐Azo and Tp‐TTA ) with micrometer thickness. The as‐synthesized films are porous (SABET=2033 m2 g?1 for Tp‐Azo ) and chemically stable. These COFs have distinct size cut‐offs (ca. 2.7 and ca. 1.6 nm for Tp‐Azo and Tp‐TTA , respectively), which allow the size‐selective separation of gold nanoparticles. Unlike, other conventional membranes, the durable structure of the COF films allow for excellent recyclability (up to 4 consecutive cycles) and easy recovery of the gold nanoparticles from the solution. 相似文献
110.
Mandal U Ghosh S Dey S Adhikari A Bhattacharyya K 《The Journal of chemical physics》2008,128(16):164505
Ultrafast photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to coumarin dyes is studied in the micelle and the gel phase of a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20) (Pluronic P123) by picosecond and femtosecond emission spectroscopies. The rate of PET in a P123 micelle and gel is found to be nonexponential and faster than the slow components of solvation dynamics. In a P123 micelle and gel, PET occurs on multiple time scales ranging from a subpicosecond time scale to a few nanoseconds. In the gel phase, the highest rate constant (9.3 x 10(9) M(-1) s(-1)) of ET for C152 is about two times higher than that (3.8 x 10(9) M(-1) s(-1)) observed in micelle phase. The ultrafast components of electron transfer (ET) exhibits a bell shaped dependence with the free energy change which is similar to the Marcus inversion. Possible reasons for slower PET in P123 micelle compared to other micelles and relative to P123 gel are discussed. 相似文献