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991.
The aeroassisted flight experiment (AFE) refers to a spacecraft to be launched and then recovered by the space shuttle in 1994. It simulates a transfer from a geosynchronous Earth orbit (GEO) to a low Earth orbit (LEO). Specifically, the AFE spacecraft is released from the space shuttle and is accelerated by means of a solid rocket motor toward Earth, so as to achieve atmospheric entry conditions close to those of a spacecraft returning from GEO. Following the atmospheric pass, the AFE spacecraft ascends to the specified LEO via an intermediate parking Earth orbit (PEO). The final maneuver includes the rendezvous with and the capture by the space shuttle. The entry and exit orbital planes of the AFE spacecraft are identical with the orbital plane of the space shuttle. In this paper, with reference to the AFE spacecraft, an actual GEO-to-LEO transfer is considered and optimal trajectories are determined by minimizing the total characteristic velocity. The optimization is performed with respect to the time history of the controls (angle of attack and angle of bank), the entry path inclination and the flight time being free. Two transfer maneuvers are considered: (DA) direct ascent to LEO; (IA) indirect ascent to LEO via PEO. While the motion of the AFE spacecraft in a 3D-space is described by a system of six ODEs, substantial simplifications are possible if one exploits these facts: (i) the instantaneous orbital plane is nearly identical with the initial orbital plane; (ii) the bank angle is small; and (iii) the Earth's angular velocity is relatively small. Under these assumptions, the complete system can be decoupled into two subsystems, one describing the longitudinal motion and one describing the lateral motion. The angle of attack history, the entry path inclination, and the flight time are determined via the longitudinal motion subsystem; in this subsystem, the total characteristic velocity is minimized subject to the specified LEO requirement. The angle of bank history is determined via the lateral motion subsystem; in this subsystem, the difference between the instantaneous bank angle and a constant bank angle is minimized in the least square sense subject to the specified orbital inclination requirement. It is shown that both the angle of attack and the angle of bank are constant. This result has considerable importance in the design of nominal trajectories to be used in the guidance of AFE and AOT vehicles. 相似文献
992.
A. Miele M. Ciarcià M. W. Weeks 《Journal of Optimization Theory and Applications》2007,132(3):377-400
In a previous paper of Miele et al. (J. Optim. Theory Appl. 132(1), 2007), we employed the single-subarc sequential gradient-restoration algorithm to optimize the three-dimensional rendezvous between
a target spacecraft in a planar circular orbit and a chaser spacecraft with an initial separation distance and separation
velocity. The achieved continuous solutions are characterized by two, three, or four subarcs depending on the performance
index (time, fuel) and the constraints. In this paper, based on the solutions in Miele et al. (J. Optim. Theory Appl. 132(1), 2007), we employ the multiple-subarc sequential gradient-restoration algorithm to produce pieced guidance trajectories implementable
in real time via constant control components. In other words, in this investigation, we force the controls to behave as parameters
in each subarc. With the above understanding, we investigate four problems: (P1) minimum time, fuel free; (P2) minimum fuel,
time free; (P3) minimum time, fuel given; (P4) minimum fuel, time given.
Problem P1 results in a two-subarc solution, each subarc with constant controls: a max-thrust accelerating subarc followed
by a max-thrust braking subarc. Problem P2 results in a four-subarc solution, each subarc with constant controls: an initial
coasting subarc, followed by a max-thrust braking subarc, followed by another coasting subarc, followed by another max-thrust
braking subarc. Problems P3 and P4 result in two, three, or four-subarc solutions depending on the performance index and the
constraints, albeit with constant controls in each subarc.
For each of the problems studied, the performance index of the multiple-subarc pieced guidance trajectory approximates well
the performance index of the single-subarc continuous optimal trajectory of Miele et al. (J. Optim. Theory Appl. 132(1), 2007) as well as the performance index of the multiple-subarc pieced optimal trajectory: the pairwise relative differences in
performance index are less than 1/100.
This research was supported by NSF under Grant CMS-0218878. 相似文献
993.
994.
M. H. Feinberg Alice Gerbanowski D. N. Rutledge 《Accreditation and quality assurance》1997,2(2):69-75
Method validation procedure requires a strategy for collecting those validation data that are best adapted to the analytical
technique used. A flexible and general approach based on Object Linking and Embedding technology is proposed. It allows a
traceable validation strategy using modular objects which encapsulate documentation, analytical data and processing logic.
The contents of these objects are accessible through a standard user interface. This paper demonstrates how this can reduce
experiment time, simplify evaluation efforts, and increase the ease of use of validation figures of merit. An illustration
using Microsoft Visual Basic for Applications is presented, and some specific aspects are described. It consists of the evaluation
of a time domain NMR technique for determining the moisture content of foods involving a multivariate calibration step. This
study also illustrates how guidelines such as Good Validation Practices could be defined to present all validation documents
in a standardised manner.
Received: 3 September 1996 Accepted: 14 October 1996 相似文献
995.
Laser forming is a process that uses the energy of relatively high powered lasers to cause permanent deformation to components by inducing localised thermal stresses. It is envisaged that this material processing technique will find a number of commercial applications. This paper briefly discusses laser forming and the development of a basic process monitoring and control system used to overcome variability problems due to the complex nature of the lasers themselves and the manner in which they interact with material. It then goes on to show how the basic control system was modified, using increased feedback data sampling, time delays and a modified control algorithm which takes account of the forming rate in addition to the error. The effect of these developments is then illustrated by a series of tests which show the modifications significantly improve process tolerances. 相似文献
996.
Aqueous solutions of a well-defined poly(N-isopropylacrylamide-co-sodium 2-acrylamido-methylpropanesulfonate) (NIPAM/NaAMPS in a 95/5 molar ratio) have been investigated by means of small-angle
neutron scattering (SANS) and rheological experiments as a function of temperature ( 25°
C
T
60°
C) and polymer concentration ( 0.5wt%
C
12wt%). The solutions remain optically transparent and isotropic over the whole temperature range, in contrast with the homopolyNIPAM
which precipitates above its lower critical solution temperature (LCST = 32°
C). Upon addition of salt, the systems undergo a micro-macrophase separation. At temperatures above 45°
C, the SANS spectra exhibit a sharp peak at a scattering wave vector, q
max, which increases slightly with temperature. At high temperature ( T∼ 60°
C), the scattered intensity follows a power law I(q) ∼q
-4 in the asymptotic regime, characteristic of two-density media with sharp interfaces, and q
max is found to vary with polymer concentration as q
max∼C
0.22. Estimates of the typical sizes give values between 40 ? and 200 ?. These results provide a strong evidence of a thermally
induced microphase separation, which is corroborated by the very sharp increases of the viscosity (over 2 decades) and of
the stress relaxation time of the solutions, occurring in the temperature range where the scattering peak is observed. The
results are discussed and compared with the theoretical models proposed for weakly charged polyelectrolytes in a poor solvent.
Received 1 October 2001 相似文献
997.
S. Tosto 《Il Nuovo Cimento D》1996,18(12):1363-1394
Summary The paper concerns a theoretical model to calculate the number of allowed states for the electrons in the diatomic molecules
through the quantum-statistical analysis of the phase space. The approach is a straightforward extension of that already discussed
in the case of many-electron atoms. Simple formulae are derived that enable to calculate the dissociation energy, bond length
and vibrational frequency of diatomic molecules in a good agreement with the experimental data. 相似文献
998.
Canio Noce 《Il Nuovo Cimento D》1991,13(3):343-361
Summary The effects of the shape of free conduction density of states on the physical quantities for the periodic Anderson model have
been investigated. 相似文献
999.
Nicolay Stanoulov 《Annals of Operations Research》1994,51(3):141-162
Based on a simple outranking method of multi-attribute decision making (MADM), called Dichotomy-Cut, developed in recent years and published elsewhere, this paper describes how to reliably elicit the necessary decision knowledge for implementing various decision modes via this method and its computerized support - UNIDAS 2. The most important features of this method are outlined, namely, how to compose the preference ordered matrices and which are the basic decision rules. Special attention should be paid to specific interactive group decision behaviour of experts. Two examples plainly explain the contents discussed. 相似文献
1000.
R P Sharma 《Pramana》1988,31(3):185-195
Fabrication methods for silicon surface barrier detectors and their correlated properties which result in the production of
high resolution (< 20 keV) devices have been studied. The techniques for fabrication and testing of the detectors currently
employed at our Centre are presented. An FWHM of 14keV for 5·486 MeV241Am α has been achieved. Our results are therefore comparable with the best in the world. 相似文献