全文获取类型
收费全文 | 2067篇 |
免费 | 73篇 |
国内免费 | 37篇 |
专业分类
化学 | 1159篇 |
晶体学 | 35篇 |
力学 | 65篇 |
数学 | 375篇 |
物理学 | 543篇 |
出版年
2023年 | 16篇 |
2022年 | 20篇 |
2021年 | 37篇 |
2020年 | 37篇 |
2019年 | 34篇 |
2018年 | 26篇 |
2017年 | 27篇 |
2016年 | 53篇 |
2015年 | 36篇 |
2014年 | 53篇 |
2013年 | 112篇 |
2012年 | 145篇 |
2011年 | 154篇 |
2010年 | 92篇 |
2009年 | 67篇 |
2008年 | 113篇 |
2007年 | 94篇 |
2006年 | 105篇 |
2005年 | 81篇 |
2004年 | 79篇 |
2003年 | 66篇 |
2002年 | 56篇 |
2001年 | 46篇 |
2000年 | 53篇 |
1999年 | 48篇 |
1998年 | 22篇 |
1997年 | 36篇 |
1996年 | 20篇 |
1995年 | 33篇 |
1994年 | 28篇 |
1993年 | 38篇 |
1992年 | 46篇 |
1991年 | 34篇 |
1990年 | 31篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 26篇 |
1984年 | 22篇 |
1983年 | 6篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 8篇 |
1979年 | 17篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1975年 | 5篇 |
1974年 | 11篇 |
1973年 | 7篇 |
排序方式: 共有2177条查询结果,搜索用时 0 毫秒
81.
H. S. Tan M. K. Han P. Y. Hu J. H. Zheng S. C. Ng H. Gong 《Applied Physics A: Materials Science & Processing》1994,59(3):245-251
An electron trap having an energy level of 0.14 eV from the conduction band edge was found in the bulk of copper-diffused VPE-grown n-GaAs0.6P0.4 by conventional DLTS measurements and by pulse-duration dependent capacitance amplitude measurements. The capture cross section at room temperature is about 1.0×10–21 cm2 and has a weak temperature dependence. These properties are attributed to a non-repulsive center having a capturing mechanism which involves multiphonon emission processes with hardly any lattice relaxation. Evolution of the spatial distributions of the traps with time under junction electric field were studied. The results suggest that the trap is positively charged and has a high diffusivity under electric field. The center can thus be identified as positively charged interstitial copper ion rather than some form of copper complexes. 相似文献
82.
U. Milkau M. L. Begemann-Blaich E. -M. Eckert G. Imme P. Kreutz A. Kühmichel M. Lattuada U. Lynen C. Mazur W. F. J. Müller J. B. Natowitz C. Ngô J. Pochodzalla G. Raciti M. Ribrag H. Sann W. Trautmann R. Trockel 《Zeitschrift für Physik A Hadrons and Nuclei》1993,346(3):227-236
Inclusive cross sections of intermediate mass fragments from the reaction84Kr+197Au atE/A=35 MeV were measured over the range 8°≦Θ lab≦70° with a low detection threshold. A moving-source parameterization was used to fit the double-differential cross sections. The integrated cross section for fragment production exceeds the total reaction cross section thus indicating a large probability for multi-fragment processes. The deduced large temperature parameters can be explained by assuming emission from a rotating source. From the comparison to reactions with12C and40Ar projectiles at E/A=30 MeV a systematics of inclusive fragment production as a function of the projectile mass is obtained. 相似文献
83.
J.?KongEmail author D.Y.?Tang S.P.?Ng B.?Zhao L.J.?Qin X.L.?Meng 《Applied physics. B, Lasers and optics》2004,79(2):203-206
We report on a diode end-pumped passively mode-locked Nd:GdVO4 laser. By using a GaAs wafer simultaneously as the saturable absorber and the output coupler, stable continuous-wave mode locking was achieved. The pulse width was measured to be 18.9 picoseconds at a repetition rate of 370 MHz. The most remarkable property of the laser is that its repetition rate can be changed from 370 MHz to 3.348 GHz by simply changing the cavity length. An average output power of 3.46 W at a 3.348 GHz repetition rate was obtained with a 14 W pump power. To our knowledge, this is the first demonstration of a passively mode-locked Nd:GdVO4 laser using a GaAs wafer as the saturable absorber. PACS 42.55.Rz; 42.60.Fc; 42.55.Xi. 相似文献
84.
85.
86.
Carmine Coluccini Yoke Mooi Ng Yves Ira A. Reyes Hsin-Yi Tiffany Chen Yit Lung Khung 《Molecules (Basel, Switzerland)》2020,25(22)
In this paper, a modified Cyclotriveratrylene was synthesized and linked to a branched Polyethylenimine, and this unique polymeric material was subsequently examined as a potential supramolecular carrier for Doxorubicin. Spectroscopic analysis in different solvents had shown that Doxorubicin was coordinated within the hollow-shaped unit of the armed Cyclotriveratrylene, and the nature of the host–guest complex revealed intrinsic Van der Waals interactions and hydrogen bonding between the host and guest. The strongest interaction was detected in water because of the hydrophobic effect shared between the aromatic groups of the Doxorubicin and Cyclotriveratrylene unit. Density functional theory calculations had also confirmed that in the most stable coordination of Doxorubicin with the cross-linked polymer, the aromatic rings of the Doxorubicin were localized toward the Cyclotriveratrylene core, while its aliphatic chains aligned closer with amino groups, thus forming a compact supramolecular assembly that may confer a shielding effect on Doxorubicin. These observations had emphasized the importance of supramolecular considerations when designing a novel drug delivery platform. 相似文献
87.
Raymond H. Chan Michael K. Ng Robert J. Plemmons 《Numerical Linear Algebra with Applications》1996,3(1):45-64
In this paper, we propose a method to generalize Strang's circulant preconditioner for arbitrary n-by-n matrices An. The th column of our circulant preconditioner Sn is equal to the th column of the given matrix An. Thus if An is a square Toeplitz matrix, then Sn is just the Strang circulant preconditioner. When Sn is not Hermitian, our circulant preconditioner can be defined as . This construction is similar to the forward-backward projection method used in constructing preconditioners for tomographic inversion problems in medical imaging. We show that if the matrix An has decaying coefficients away from the main diagonal, then is a good preconditioner for An. Comparisons of our preconditioner with other circulant-based preconditioners are carried out for some 1-D Toeplitz least squares problems: min ∥ b - Ax∥2. Preliminary numerical results show that our preconditioner performs quite well, in comparison to other circulant preconditioners. Promising test results are also reported for a 2-D deconvolution problem arising in ground-based atmospheric imaging. 相似文献
88.
We study the perturbation property of best approximation to a set defined by an abstract nonlinear constraint system. We show that, at a normal point, the perturbation property of best approximation is equivalent to an equality expressed in terms of normal cones. This equality is related to the strong conical hull intersection property. Our results generalize many known results in the literature on perturbation property of best approximation established for a set defined by a finite system of linear/nonlinear inequalities. The connection to minimization problem is considered.The authors thank the referees for valuable suggestions.K.F. Ng - This author was partially supported by Grant A0324638 from the National Natural Science Foundation of China and Grants (2001) 01GY051-66 and SZD0406 from Sichuan Province.
Y.R. He -This author was supported by a Direct Grant (CUHK) and an Earmarked Grant from the Research Grant Council of Hong Kong. 相似文献
89.
Ciwei Dong Bin Shen Pui-Sze Chow Liu Yang Chi To Ng 《Annals of Operations Research》2016,240(2):509-531
Carbon emission abatement is a hot topic in environmental sustainability and cap-and-trade regulation is regarded as an effective way to reduce the carbon emission. According to the real industrial practices, sustainable product implies that its production processes facilitate to reduce the carbon emission and has a positive response in market demand. In this paper, we study the sustainability investment on sustainable product with emission regulation consideration for decentralized and centralized supply chains. We first examine the order quantity of the retailer and sustainability investment of the manufacturer for the decentralized supply chain with one retailer and one manufacturer. After that, we extend our study to the centralized case where we determine the production quantity and sustainability investment for the whole supply chain. We derive the optimal order quantity (or production quantity) and sustainability investment, and find that the sustainability investment efficiency has a significant impact on the optimal solutions. Further, we conduct numerical studies and find surprisingly that the order quantity may be increasing in the wholesale price due to the effects of the sustainability and emission consideration. Moreover, we investigate the achievability of supply chain coordination by various contracts, and find that only revenue sharing contract can coordinate the supply chain whereas the buyback contract and two-part tariff contract cannot. Important insights and managerial implications are discussed. 相似文献
90.
A vertex labeling f : V → Z2 of a simple graph G = (V, E) induces two edge labelings f+ , f*: E → Z2 defined by f+ (uv) = f(u)+f(v) and f*(uv) = f(u)f(v). For each i∈Z2 , let vf(i) = |{v ∈ V : f(v) = i}|, e+f(i) = |{e ∈ E : f+(e) = i}| and e*f(i)=|{e∈E:f*(e)=i}|. We call f friendly if |vf(0)-vf(1)|≤ 1. The friendly index set and the product-cordial index set of G are defined as the sets{|e+f(0)-e+f(1)|:f is friendly} and {|e*f(0)-e*f(1)| : f is friendly}. In this paper we study and determine the connection between the friendly index sets and product-cordial index sets of 2-regular graphs and generalized wheel graphs. 相似文献