全文获取类型
收费全文 | 769篇 |
免费 | 81篇 |
国内免费 | 24篇 |
专业分类
化学 | 76篇 |
晶体学 | 2篇 |
力学 | 406篇 |
综合类 | 12篇 |
数学 | 207篇 |
物理学 | 171篇 |
出版年
2023年 | 5篇 |
2022年 | 18篇 |
2021年 | 12篇 |
2020年 | 12篇 |
2019年 | 7篇 |
2018年 | 14篇 |
2017年 | 19篇 |
2016年 | 17篇 |
2015年 | 23篇 |
2014年 | 42篇 |
2013年 | 40篇 |
2012年 | 23篇 |
2011年 | 37篇 |
2010年 | 23篇 |
2009年 | 27篇 |
2008年 | 47篇 |
2007年 | 61篇 |
2006年 | 52篇 |
2005年 | 49篇 |
2004年 | 28篇 |
2003年 | 40篇 |
2002年 | 27篇 |
2001年 | 32篇 |
2000年 | 24篇 |
1999年 | 20篇 |
1998年 | 10篇 |
1997年 | 29篇 |
1996年 | 20篇 |
1995年 | 12篇 |
1994年 | 17篇 |
1993年 | 11篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1972年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有874条查询结果,搜索用时 15 毫秒
181.
利用R/S分析研究了农业发展的总趋势.农业发展的长期变化过程既带有趋势变化成分,又带有周期变化成分,还带有随机变化成分,因而根据趋势变化分析、周期变化分析和随机变化分析集成的方法来预测农业发展是可行的,提出的集成预测模型的拟合误差比单一模型的拟合误差小,预测效果比较好,是农业发展预测的一条比较有效的途径. 相似文献
182.
Lan Jin Yonghui Deng Jianhua Hu Changchun Wang 《Journal of polymer science. Part A, Polymer chemistry》2004,42(23):6081-6088
A series of polystyrene (PS)/poly(vinyl acetate) (PVAc) crosslinked particles (240, 210, or 90 nm) with different concentrations of PS (75, 50, or 25 wt %) were prepared by soap‐free emulsion polymerization. Based on the crosslinked polymer particles, three series of monodisperse core–shell particles with pH‐sensitive poly[2‐(dimethylamino)ethyl methacrylate] (PDMAEMA) shells were synthesized by oxyanionic polymerization. During the oxyanionic polymerization, the acetate groups of PVAc were hydrolyzed, and the hydroxyl groups that formed on the surfaces of the particles, acting as initiators, were transferred to ? O?K+ by DMSO?K+ (where DMSO is dimethyl sulfoxide) at the same time; then, ? O?K+ initiated the polymerization of 2‐(dimethylamino)ethyl methacrylate. 1H NMR and Fourier transform infrared studies confirmed the existence of PDMAEMA shells, and the contents of PDMAEMA were measured by elemental analysis. Because the PDMAEMA chain could be protonated at a low pH, these core–shell particles could adsorb negatively charged modified magnetite particles, and at higher pHs, the magnetite particles could be released again; this process was reversible. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6081–6088, 2004 相似文献
183.
The paper proposes a new approach of predicting the bifurcation points of elastic-plastic buckling of plates and shells, which
is obtained from the natural combination of the Lyaponov's dynamic criterion on stability and the modified adaptive Dynamic
Relaxation (maDR) method developed recently by the authors. This new method can overcome the difficulties in the applications
of the dynamic criterion. Numerical results show that the theoretically predicted bifurcation points are in very good agreement
with the corresponding experimental ones. The paper also provides a new means for further research on the plastic buckling
paradox of plates and shells. 相似文献
184.
185.
G. J. Zalmai 《Journal of Global Optimization》2006,36(1):51-85
The purpose of this paper is to develop a fairly large number of sets of global semiparametric sufficient efficiency conditions under various generalized (η, ρ)-invexity assumptions for a multiobjective fractional programming problem involving arbitrary norms. 相似文献
186.
Construction of improved estimators in multiparameter estimation for continuous exponential families
A unified theory of simultaneous estimation of parameters for the continuous exponential family is presented. Estimators are constructed that improve on the standard ones (the maximum likelihood, UMVUE or best invariant estimator). These improved estimators shift the standard ones towards possibly non-zero points or data based points. 相似文献
187.
In this paper we consider the eigenvalue problem for piezoelectric shallow shells and we show that, as the thickness of the
shell goes to zero, the eigensolutions of the three-dimensional piezoelectric shells converge to the eigensolutions of a twodimensional
eigenvalue problem. 相似文献
188.
By using Donnell's simplication and starting from the displacement type equations of conical shells, and introducing a displacement functionU(s,,) (In the limit case, it will be reduced to cylindrical shell displacement function introduced by V. S. Vlasov) and a generalized loadq,(s,,),the equations of conical shells are changed into an eighth—order solvable partial differential equation about the displacement functionU(s,,). As a special case, the general bending problem of conical shells on Winkler foundation has been studied. Detailed numerical results and boundary coefficients for edge unit loads are obtained.The project supported by the National Natural Science Foundation of China. 相似文献
189.
《Journal of computational chemistry》2018,39(19):1226-1235
The structural nature of the solvation shells of an iodate ion, which is known to be a polyoxy‐anion with a large cationic centre, is investigated by means of Born–Oppenheimer molecular dynamics (BOMD) simulations using BLYP and the dispersion corrected BLYP‐D3 functionals. The iodate ion is found to have two distinct solvation regions around the positively charged iodine (iodine solvation shell or ISS) and the negatively charged oxygens (oxygen solvation shell or OSS). We have looked at the spatial, orientational, and hydrogen bond distributions of water in the two solvation regions. It is found that the water orientational profile in the ISS is typical of a cation hydration shell. The hydrogen bonded structure of water in the OSS is found to be very similar to that of the bulk water structure. Thus, the iodate ion essentially behaves like a positively charged iodine ion in water as if there is no anionic part. This explains why the cationic character of the iodate ion was prominently seen in earlier studies. The arrangement of water molecules in the two solvation shells and in the intervening regions around the iodate ion is further resolved by looking at structural cross‐correlations. The electronic properties of the solvation shells are also looked at by calculating the solute–solvent orbital overlap and dipole moments of the solute and solvation shell water. We have also performed BOMD simulations of iodate ion‐water clusters at experimentally relevant conditions. The simulation results are found to be in agreement with experimental results. © 2018 Wiley Periodicals, Inc. 相似文献
190.
Ramon Carbó-Dorca 《Journal of mathematical chemistry》2002,32(2):201-223
Vector semispaces are studied from a realistic way with the intention to define a natural metric, adapted to their peculiar structure, which reside on the essential positive definiteness of their elements. From this point of view, Minkowski norms allow classifying semispaces in shells, that is: subsets where all the vector elements possess the same norm values. Shell structure appears to be a possible disjoint partition of any semispace and so shells become equivalence classes Then, the unit shell appears to be the core of the semispace homothetic construction as well as the origin of the semispace metrics. Minkowski or root scalar products permit to connect two or more semispace elements and conduct towards generalized definitions of Pth order root distances and cosines. Finally, the unit shell of a given semispace, in company of both Boolean tagged sets, inward matrix products and with the aid of the matrix signatures as well, it is seen as the seed to construct any arbitrary element of the semispace connected vector space. Finite and infinite dimensional vector spaces application examples are provided along the work discussion. 相似文献