全文获取类型
收费全文 | 8339篇 |
免费 | 247篇 |
国内免费 | 42篇 |
专业分类
化学 | 5833篇 |
晶体学 | 40篇 |
力学 | 209篇 |
数学 | 1107篇 |
物理学 | 1439篇 |
出版年
2023年 | 46篇 |
2022年 | 56篇 |
2021年 | 66篇 |
2020年 | 93篇 |
2019年 | 99篇 |
2018年 | 72篇 |
2017年 | 61篇 |
2016年 | 184篇 |
2015年 | 152篇 |
2014年 | 174篇 |
2013年 | 370篇 |
2012年 | 460篇 |
2011年 | 532篇 |
2010年 | 284篇 |
2009年 | 233篇 |
2008年 | 516篇 |
2007年 | 546篇 |
2006年 | 548篇 |
2005年 | 474篇 |
2004年 | 427篇 |
2003年 | 367篇 |
2002年 | 346篇 |
2001年 | 149篇 |
2000年 | 106篇 |
1999年 | 66篇 |
1998年 | 82篇 |
1997年 | 100篇 |
1996年 | 133篇 |
1995年 | 126篇 |
1994年 | 105篇 |
1993年 | 145篇 |
1992年 | 96篇 |
1991年 | 78篇 |
1990年 | 96篇 |
1989年 | 74篇 |
1988年 | 64篇 |
1987年 | 65篇 |
1986年 | 52篇 |
1985年 | 90篇 |
1984年 | 81篇 |
1983年 | 57篇 |
1982年 | 87篇 |
1981年 | 90篇 |
1980年 | 69篇 |
1979年 | 70篇 |
1978年 | 68篇 |
1977年 | 50篇 |
1976年 | 69篇 |
1975年 | 47篇 |
1973年 | 36篇 |
排序方式: 共有8628条查询结果,搜索用时 17 毫秒
791.
The effect of different storage temperatures on the physical properties of pectin solutions and gels
Gordon A. Morris Jonathan Castile Gary G. Adams Stephen E. Harding 《Polymer Degradation and Stability》2010,95(12):2670-2673
The stability (in terms of viscosity and gel strength) of pectin solutions and gels potentially plays an important role in their behaviour and functional properties in a wide range of applications and therefore any changes over time must be understood. The gel strength of pectin gels and intrinsic viscosity of pectin solutions at different temperatures (4 °C, 25 °C and 40 °C) have been investigated using a “rolling ball” viscometer and a texture analyser respectively. Both the intrinsic viscosity ([η]) and gel strength decrease with increased storage time, although this more pronounced at elevated temperatures. The changes in intrinsic viscosity with storage time and temperature were used to determine the depolymerisation constant (k). Pectin storage conditions and particularly temperature have an influence on depolymerisation, particularly elevated storage temperatures, but whether or not this will be detrimental to its intended application will depend on the functional significance of the changes that occur. In this case based on the previous diffusion studies on a model drug (paracetamol) we conclude that the decreases in viscosity and gel strength within the range observed have no detrimental effect on the drug release properties. 相似文献
792.
793.
794.
Jerald K. Rasmussen Steven M. Heilmann Larry R. Krepski Howell K. Smith Alan R. Katritzky Kumars Sakizadeh 《Journal of polymer science. Part A, Polymer chemistry》1986,24(11):2739-2747
The reaction of bisazlactones (2-oxazolin-5-ones) with primary diamines containing additional secondary or tertiary amine functionality (e.g., diethylenetriamine, triethylenetetramine, or N-methyliminobispropylamine) readily produces polyamides which serve as precursors to a new class of heterocyclic polymers. Thermal cyclodehydration takes place under relatively mild conditions (180–200°C) to produce water-soluble polymers containing the 2-imidazolin-5-one heterocycle. Model reactions have been studied to verify this mode of cyclization and confirm the proposed polymer structure. 相似文献
795.
796.
A new method for departure point determination on Cartesian grids, the semi‐analytical upwind path line tracing (SUT) method, is presented and compared to two typical departure point determination methods used in semi‐Lagrangian advection schemes, the Euler method and the four‐step Runge–Kutta method. Rigorous comparisons of the three methods were conducted for a severely curving hypothetical flow field and for advective transport in the rotation of a Gaussian concentration hill. The SUT method was shown to have equivalent accuracy to the Runge–Kutta method but with significantly improved computational efficiency. Depending on the case being simulated, the SUT method provides either far greater or equivalent computational efficiency and more certain accuracy than the Euler method. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
797.
An optimal control problem for an elliptic obstacle variational inequality is considered. The obstacle is taken to be the
control and the solution to the obstacle problem is taken to be the state. The goal is to find the optimal obstacle from H
1
0
(Ω) so that the state is close to the desired profile while the H
1
(Ω) norm of the obstacle is not too large. Existence, uniqueness, and regularity as well as some characterizations of the optimal
pairs are established.
Accepted 11 September 1996 相似文献
798.
Jones Steven E. Shelton David S. Steven Turley R. Jeannette Lawler M. Allred David D. 《Hyperfine Interactions》1996,101(1):695-698
We show that Raman spectral lines from H2, D2, T2, HD, HT and DT are readily resolved, permitting an effective means to analyze isotopic hydrogen mixtures used in muon-catalyzed fusion experiments. We propose a Raman spectrographic system to allow for real-time analysis of targets involving all three isotopes of hydrogen. 相似文献
799.
Steven Prestwich 《Annals of Operations Research》2007,156(1):129-141
Branch-and-bound uses relaxation to prune search trees but sometimes scales poorly to large problems. Conversely, local search
often scales well but may be unable to find optimal solutions. Both phenomena occur in the construction of low-autocorrelation
binary sequences (LABS), a problem arising in communication engineering. This paper proposes a hybrid approach to optimization:
using relaxation to prune local search spaces. An implementation gives very competitive results, showing the feasibility of
the approach. 相似文献
800.
Enhanced locking region in injection-locked two-section lasers 总被引:1,自引:0,他引:1
In this Letter, we investigate the criteria for achieving an enhanced stable locking region in the parameter space of injected field Einj and frequency detuning ω in two-section semiconductor lasers subject to optical injection. Using the rate equations, a stability analysis is performed and the conditions for an enhanced locking region are deduced from the requirement for the average linewidth enhancement factor appearing in the locking-bandwidth equation to be close to zero. We calculate a stability map for an injection-locked two-section laser with an almost-symmetrical locking region that is greatly enhanced compared to a single-section injection-locked laser with identical material and control parameters. 相似文献