全文获取类型
收费全文 | 336篇 |
免费 | 1篇 |
国内免费 | 2篇 |
专业分类
化学 | 194篇 |
晶体学 | 3篇 |
力学 | 9篇 |
数学 | 101篇 |
物理学 | 32篇 |
出版年
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 6篇 |
2014年 | 8篇 |
2013年 | 17篇 |
2012年 | 20篇 |
2011年 | 19篇 |
2010年 | 3篇 |
2009年 | 10篇 |
2008年 | 14篇 |
2007年 | 11篇 |
2006年 | 11篇 |
2005年 | 12篇 |
2004年 | 11篇 |
2003年 | 13篇 |
2002年 | 10篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1981年 | 11篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1975年 | 15篇 |
1974年 | 5篇 |
1973年 | 12篇 |
1972年 | 4篇 |
1970年 | 2篇 |
1965年 | 3篇 |
1959年 | 1篇 |
1948年 | 1篇 |
排序方式: 共有339条查询结果,搜索用时 31 毫秒
61.
Mark E. Ridgeway Jeremy J. Wolff Joshua A. Silveira Cheng Lin Catherine E. Costello Melvin A. Park 《International Journal for Ion Mobility Spectrometry》2016,19(2-3):77-85
Analysis of molecules by ion mobility spectrometry coupled with mass spectrometry (IMS-MS) provides chemical information on the three dimensional structure and mass of the molecules. The coupling of ion mobility to trapping mass spectrometers has historically been challenging due to the large differences in analysis time between the two devices. In this paper we present a modification of the trapped ion mobility (TIMS) analysis scheme termed “Gated TIMS” that allows efficient coupling to a Fourier Transform Ion Cyclotron Resonance (FT-ICR) analyzer. Analyses of standard compounds and the influence of source conditions on the TIMS distributions produced by ion mobility spectra of labile ubiquitin protein ions are presented. Ion mobility resolving powers up to 100 are observed. Measured collisional cross sections of ubiquitin ions are in excellent qualitative and quantitative agreement to previous measurements. Gated TIMS FT-ICR produces results comparable to those acquired using TIMS/time-of-flight MS instrument platforms as well as numerous drift tube IMS-MS studies published in the literature. 相似文献
62.
D'Souza F Gadde S Islam DM Wijesinghe CA Schumacher AL Zandler ME Araki Y Ito O 《The journal of physical chemistry. A》2007,111(35):8552-8560
A new concept of charge stabilization via delocalization of the pi-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin pi-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the delocalization of the pi-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates. 相似文献
63.
Daniel J. Schell Mark F. Ruth Melvin P. Tucker 《Applied biochemistry and biotechnology》1999,77(1-3):67-81
Glucose yield from the enzymatic hydrolysis of cellulose was investigated as a function of cellulase enzyme loading (7–36
filter paper units [FPU]/g cellulose) and solids concentration (7–18% total solids) for up to 72 h on dilute sulfuric-acid
pretreated Douglas Fir. The saccharification was performed on whole hydrolysate with no separation or washing of the solids.
Enzyme loading had a significant effect on glucose yield; solids concentration had a much smaller effect even at higher glucose
concentrations. The data were used to generate an empirical model for glucose yield, and to fit parameters of a cellulose
hydrolysis kinetic model. Both models could be used for economic evaluation of a separate hydrolysis and fermentation process. 相似文献
64.
The present study was designed to examine changes in vocal intensity following unilateral posteroventral pallidotomy (PVP) in a large sample of speakers with Parkinson's disease (PD) that exhibited a range of hypokinetic dysarthria. Twenty-five persons with PD were recorded using a variety of speech tasks, once prior to and once following PVP. The pre-PVP vocal SPL was subtracted from the post-PVP vocal SPL to derive a relative change in vocal SPL. Mildly dysarthric participants had significantly greater relative increases in vocal SPL following PVP than either moderately or severely dysarthric participants who had reduced vocal SPL following PVP. If future results follow those observed in the present data, mildly dysarthric Parkinson's patients may benefit most from unilateral PVP perhaps due to less overall destruction of the basal ganglia sensorimotor control circuits involved in oral facial functions, thus increasing the chances to observe improvements postsurgery. 相似文献
65.
As is well-known to those familiar with the Gillman-Jerison text on the ring C(X) of real-valued continuous functions on a topological space (which we may assume is a Tychonoff space), every prime ideal is contained in a unique maximal ideal. In case X is compact, each maximal ideal is of the form M p for some ${p \in X}$ and consists of all ${f \in C(X)}$ such that f(p) = 0, while the intersection of all minimal prime ideals contained in M p is the set of all continuous functions which vanish on a neighborhood of p. In this paper, we reverse some inclusions and study the union of all of the minimal prime ideals contained in M p ; particularly in the case when this set-theoretic union is all of M p . When this occurs, we call X a UMP-space. By making use of the well-known theorem of Gelfand and Kolmogoroff, we obtain new results without assuming that X is compact. It turns out that all UMP-spaces have the property that each of its nonempty zero-sets has nonempty interior. That is, X is an almost P-space. But this condition is far from sufficient. 相似文献
66.
The numerical analysis of variational integrators relies on variational error analysis, which relates the order of accuracy
of a variational integrator with the order of approximation of the exact discrete Lagrangian by a computable discrete Lagrangian.
The exact discrete Lagrangian can either be characterized variationally, or in terms of Jacobi’s solution of the Hamilton-Jacobi
equation. These two characterizations lead to the Galerkin and shooting constructions for discrete Lagrangians, which depend
on a choice of a numerical quadrature formula, together with either a finite-dimensional function space or a one-step method.
We prove that the properties of the quadrature formula, finite-dimensional function space, and underlying one-step method
determine the order of accuracy and momentum-conservation properties of the associated variational integrators. We also illustrate
these systematic methods for constructing variational integrators with numerical examples. 相似文献
67.
Gan Jet Hong Melvin Qing-Qing Ni Toshiaki Natsuki 《Applied Physics A: Materials Science & Processing》2014,117(4):2043-2050
We fabricated an actuator that was made from polyurethane (PU) with carbon nanotubes (CNTs) as the filler. To improve the dispersion of the CNTs, a mild hydrothermal treatment was carried out. Carboxyl and hydroxyl groups were introduced to the surface of the CNTs, and they were found to be highly dispersed in polar solvents such as dimethylformamide. To evaluate these films, we mainly focused on electrical properties, such as dielectric spectroscopy, space charge measurements, and actuator behavior. We found that the PU/CNTs film bents toward the cathode when an electric field was applied, and it reverted to its original position when the electric field was removed. Upon the inclusion of the CNTs as the filler for the polymer, the electrical properties of the films improved significantly. The highly polarized films had a high relative permittivity, and this produced a higher Maxwell pressure, which assisted the actuation. A high accumulated charge density was observed from space charge measurements in some of the films, and this explains the bending direction and the actuation mechanism. 相似文献
68.
El-Khouly ME Amin AN Zandler ME Fukuzumi S D'Souza F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(17):5239-5247
A molecular dyad and triad, comprised of a known photosensitizer, BF(2)-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF(2)-chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular-clip-type structure for the triad, in which the BDP and ADP entities are separated by about 14 ? with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge-separated states. Such calculations revealed a charge separation from the singlet excited BDP ((1)BDP*) to ADP (BDP(.+)-ADP(.-)) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from (1)BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to (1)ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10(11) s(-1)) in these molecular systems. The kinetics of electron transfer via (1)ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge-separation process from BDP to (1)ADP*. These findings suggest the potential of the examined ADP-BDP molecules to be efficient photosynthetic antenna and reaction center models. 相似文献
69.
Melvin R Euerby Matthew James Bengt‐Olof Axelsson Olof Rosén Patrik Petersson 《Journal of separation science》2012,35(19):2592-2598
The validity of the extended Tanaka column characterization procedure against the retention behavior of 101 analytes of widely differing properties chromatographed on five differing stationary phase chemistries has been established using a chemometric technique called principal component analysis (PCA). It was concluded that the simple and conveniently determined column characterization parameters covered the same space in the PCA loading plot as the retention times for the 101 differing analytes. This confirms that the ten column characterization parameters of the extended Tanaka protocol encode the same information as the retention times of the 101 analytes. Significant selectivity differences were observed between stationary phases and the mobile‐phase modifiers – MeOH and MeCN. PCA contribution plots served as a convenient way to highlight specific selectivity differences between stationary phases. logD values exhibited a poor correlation with retention indicating that retention in RP‐LC is not solely dictated by the analyte's hydrophobicity. The use of MeOH was found to generate greater selectivity differences with the five stationary phases than when MeCN is used. 相似文献
70.