全文获取类型
收费全文 | 6150篇 |
免费 | 542篇 |
国内免费 | 193篇 |
专业分类
化学 | 4570篇 |
晶体学 | 43篇 |
力学 | 183篇 |
综合类 | 20篇 |
数学 | 561篇 |
物理学 | 1508篇 |
出版年
2024年 | 18篇 |
2023年 | 83篇 |
2022年 | 124篇 |
2021年 | 140篇 |
2020年 | 197篇 |
2019年 | 177篇 |
2018年 | 137篇 |
2017年 | 111篇 |
2016年 | 213篇 |
2015年 | 214篇 |
2014年 | 247篇 |
2013年 | 339篇 |
2012年 | 438篇 |
2011年 | 485篇 |
2010年 | 308篇 |
2009年 | 277篇 |
2008年 | 385篇 |
2007年 | 304篇 |
2006年 | 327篇 |
2005年 | 300篇 |
2004年 | 251篇 |
2003年 | 228篇 |
2002年 | 204篇 |
2001年 | 125篇 |
2000年 | 129篇 |
1999年 | 102篇 |
1998年 | 67篇 |
1997年 | 55篇 |
1996年 | 88篇 |
1995年 | 57篇 |
1994年 | 71篇 |
1993年 | 54篇 |
1992年 | 55篇 |
1991年 | 54篇 |
1990年 | 42篇 |
1989年 | 35篇 |
1988年 | 25篇 |
1987年 | 27篇 |
1986年 | 34篇 |
1985年 | 40篇 |
1984年 | 45篇 |
1983年 | 27篇 |
1982年 | 29篇 |
1981年 | 29篇 |
1980年 | 25篇 |
1979年 | 19篇 |
1977年 | 35篇 |
1976年 | 23篇 |
1974年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有6885条查询结果,搜索用时 15 毫秒
981.
动脉狭窄对血液流速的影响 总被引:5,自引:0,他引:5
为了定量计算动脉局部狭窄对动脉管中血液流动速度的影响,本文分别对狭窄区域内定常流和非定常流动进行了求解,得出了狭窄区域内定常流和脉动流的速度表达式。本文将均匀段的流速形经Fourier分解成定常和脉动两部分,然后分别计算出狭窄区域内对应的定常和脉动流速,经Fourier合成还原成流速时域波形,同时针对各种情况将不同狭窄对不同的流速波形的作了分析比较。 相似文献
982.
In the present paper, we are interested in the propagation of Rayleigh waves in an isotropic elastic half-space coated with a thin isotropic elastic layer. The contact between the layer and the half space is assumed to be smooth. The main purpose of the paper is to establish an approximate secular equation of the wave. By using the effective boundary condition method, an approximate, yet highly accurate secular equation of fourth-order in terms of the dimensionless thickness of the layer is derived. From the secular equation obtained, an approximate formula of third-order for the velocity of Rayleigh waves is established. The approximate secular equation and the formula for the velocity obtained in this paper are potentially useful in many practical applications. 相似文献
983.
984.
许多天然存在和人工合成的氨基磷酸因具有抗菌、除草、植物生长调节等多方面的生物活性,从而使这一领域的研究十分活跃[’]。而2一咪哩琳酮和2,3一派嗓M酮也是具有除草、杀菌等多种生物活性的化合物I’],因此我们设计将a一氨基磷酸引入到2一眯吃财酮及2,3服喀二酮的结构中,设计并合成了Ill、W两类新化合物,以期找到高生物活性的化合物。1实验部分BrukerAC.P200型核磁共振仪,TMS为内标,溶剂CDC13;VGZAB-HS型质谱仪;Yanaco.CHNCORDERMT-3型元素自动分析仪;硅胶… 相似文献
985.
In this paper, a general solution for three-dimensional static piezothermoelastic problems of crystal class 6mm solids is
presented. The general solution involves four piezoelastic potential functions and a piezothermoelastic potential function,
of which four piezoelastic potential functions are governed by weighted harmonic differential equations. Compared with the
general solution given by Ashida et al., in which seven potential functions are introduced, the general solution proposed
in the present paper is more rigorously derived. Moreover, it has a simple form and is convenient for application. As an illustrative
example, the problem of a pyroelectric half-space subjected to axisymmetric heating is studied. Numerical results of displacements
stresses, electric potential and electric displacements are obtained for a cadmium selenide half-space. Thermally induced
displacements and stress distribution are compared with those obtained for the same material without piezoelectric and pyroelectric
effects.
Project supported by the National Natural Science Foundation of China (19872060). 相似文献
986.
The methanolysis of 2‐(2‐aryl ethynyl)benzonitriles is accelerated when the polar aprotic solvents are added, which could enhance the 6‐endo pathway and give isoquinolones, though the 5‐exo pathway is occupied mostly. The yields of products would also be in creased. 相似文献
987.
在PTC条件下合成了多亚甲基桥连的双巴比妥类化合物1~3,并通过了元素分析,IR及^1HNMR予以鉴定。 相似文献
988.
Wenjie Xie Siu-Chung Tam Hongru Yang Jianhui Gu Gang Zhao Yee-Loy Lam Chan-Hin Kam 《Optics & Laser Technology》1999,31(5):387
It is important for laser designers to study the characteristics of heat transfer from the laser crystal slab to the coolant in high-power DPSS laser operations. We have simulated and obtained the optimum heat transfer coefficient and coolant flow rate for our cavity design, in which the circulating coolant is maintained at a constant temperature. It has been determined that the coolant temperature and the convective heat transfer coefficient (h) are important parameters in the thermal analysis. The coefficient h is affected by the coolant flow rate, the physical properties of the laser slab and the coolant and the pumping cavity geometry. Using analytical heat transfer equation, for cooling water temperature of 300 K, the optimum flow rate for our cavity geometry is found to be 390 cm3/s, corresponding to h=5 W/cm2 K. 相似文献
989.
Alexandra Zamboulis Nicolas J. Rahier Matthias Gehringer Xavier Cattoën Gilles Niel Catherine Bied Joël J.E. Moreau Michel Wong Chi Man 《Tetrahedron: Asymmetry》2009,20(24):2880-2885
New heterogenised silica-based organocatalysts have been prepared via the sol–gel process from two silylated derivatives of l-proline, featuring either a carbamate or an ether linker. Co-gelification with variable amounts of TEOS was performed with and without porogen to yield high surface area solids. These materials were evaluated as heterogeneous phase organocatalysts for the asymmetric aldol reaction of p-nitrobenzaldehyde with acetone. 相似文献
990.
Chi Wang Yong‐Wen Cheng Yao‐Chung Hsu Tsang‐Lang Lin 《Journal of Polymer Science.Polymer Physics》2002,40(15):1626-1636
Lamellar morphology and thickness of syndiotactic polystyrene (sPS) samples melt‐crystallized at various temperatures were probed using transmission electron microscopy (TEM) and small‐angle X‐ray scattering (SAXS). In addition, the melting temperature and enthalpy of the crystallized samples were characterized with differential scanning calorimetry. Under appropriate thermal treatments, all the samples investigated in this study were crystallized into β′ crystal modification, as revealed by wide‐angle X‐ray diffraction. From the SAXS intensity profiles, a scattering peak (or shoulder) associated with lamellar features as well as the presence of anomalous scattering at the zero‐scattering vector were evidently observed. The peculiar zero‐angle scattering was successfully described by the Debye–Bueche model, and subtraction of its contribution from the raw intensity profiles was carried out to deduce the intensity profile merely associated with the lamellar feature. The lamellar thickness obtained from Lorentz‐corrected intensity profiles in this manner agrees with that measured from the TEM images, provided that the two‐phase model is applied. On the basis of the Gibbs–Thomson equation, the modest estimations of equilibrium melting temperature and the surface free energy of the fold lamellar surface are 292.7 ± 2.7 °C and 20.2 ± 2.6 erg/cm2, respectively, when lamellar thicknesses measured by TEM are applied. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1626–1636, 2002 相似文献