全文获取类型
收费全文 | 15797篇 |
免费 | 243篇 |
国内免费 | 67篇 |
专业分类
化学 | 5759篇 |
晶体学 | 358篇 |
力学 | 986篇 |
综合类 | 3篇 |
数学 | 1439篇 |
物理学 | 7562篇 |
出版年
2023年 | 61篇 |
2022年 | 180篇 |
2021年 | 152篇 |
2020年 | 86篇 |
2019年 | 102篇 |
2018年 | 197篇 |
2017年 | 168篇 |
2016年 | 282篇 |
2015年 | 199篇 |
2014年 | 339篇 |
2013年 | 693篇 |
2012年 | 871篇 |
2011年 | 1137篇 |
2010年 | 820篇 |
2009年 | 804篇 |
2008年 | 914篇 |
2007年 | 912篇 |
2006年 | 935篇 |
2005年 | 744篇 |
2004年 | 658篇 |
2003年 | 510篇 |
2002年 | 486篇 |
2001年 | 906篇 |
2000年 | 609篇 |
1999年 | 501篇 |
1998年 | 329篇 |
1997年 | 343篇 |
1996年 | 262篇 |
1995年 | 216篇 |
1994年 | 209篇 |
1993年 | 162篇 |
1992年 | 178篇 |
1991年 | 156篇 |
1990年 | 145篇 |
1989年 | 121篇 |
1988年 | 97篇 |
1987年 | 87篇 |
1986年 | 71篇 |
1985年 | 77篇 |
1984年 | 57篇 |
1983年 | 37篇 |
1982年 | 28篇 |
1981年 | 28篇 |
1980年 | 17篇 |
1978年 | 17篇 |
1977年 | 18篇 |
1976年 | 27篇 |
1975年 | 28篇 |
1974年 | 21篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Z. X. Li L. Q. Lin W. M. Zhang T. Wu J. L. Pu 《Journal of polymer science. Part A, Polymer chemistry》2006,44(3):1291-1298
A series of new polyimides were prepared via the polycondensation of (3‐amino‐2,4,6‐trimethylphenyl)‐(3′‐aminophenyl)methanone and aromatic dianhydrides, that is, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), 4,4′‐oxydiphthalic anhydride, 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride, and 2,2′‐bis(3,4‐dicarboxyphenyl) hexafluoropropane dianhydride. The structures of the polyimides were characterized by Fourier transform infrared and NMR measurements. The properties were evaluated by solubility tests, ultraviolet–visible analysis, differential scanning calorimetry, and thermogravimetric analysis. The two different meta‐position‐located amino groups with respect to the carbonyl bridge in the diamine monomer provided it with an unsymmetrical structure. This led to a restriction on the close packing of the resulting polymer chains and reduced interchain interactions, which contributed to the solubility increase. All the polyimides except that derived from BPDA had good solubility in strong aprotic solvents, such as N‐methyl‐2‐pyrrolidinone, N,N′‐dimethylacetamide, N,N‐dimethylformamide, and dimethyl sulfone, and in common organic solvents, such as cyclohexanone and chloroform. In addition, these polyimides exhibited high glass‐transition values and excellent thermal properties, with an initial thermal decomposition temperature above 470 °C and glass‐transition temperatures in the range of 280–320 °C. The polyimide films also exhibited good transparency in the visible‐light region, with transmittance higher than 80% at 450 nm and a cutoff wavelength lower than 370 nm. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1291–1298, 2006 相似文献
162.
Hydrogels have been widely used in microelectromechanical systems (MEMS) and Bio‐MEMS devices. In this article, the equilibrium swelling/deswelling of the pH‐stimulus cylindrical hydrogel in the microchannel is studied and simulated by the meshless method. The multi‐field coupling model, called multi‐effect‐coupling pH‐stimulus (MECpH) model, is presented and used to describe the chemical field, electric field, and the mechanical field involved in the problem. The partial differential equations (PDEs) describing these three fields are either nonlinear or coupled together. This multi‐field coupling and high nonlinear characteristics produce difficulties for the conventional numerical methods (e.g., the finite element method or the finite difference method), so an alternative—meshless method is developed to discretize the PDEs, and the efficient iteration technique is adopted to solve the nonlinear problem. The computational results for the swelling/deswelling diameter of the hydrogel under the different pH values are firstly compared with experimental results, and they have a good agreement. The influences of other parameters on the mechanical properties of the hydrogel are also investigated in detail. It is shown that the multi‐field coupling model and the developed meshless method are efficient, stable, and accurate for simulation of the properties of the stimuli‐sensitive hydrogel. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 326–337, 2006 相似文献
163.
A. Subramanian T.-Y. Choi L.X. Dong J. Tharian U. Sennhauser D. Poulikakos B.J. Nelson 《Applied Physics A: Materials Science & Processing》2007,89(1):133-139
We report on a novel method for local control of shell engineering in multiwalled carbon nanotubes (MWNTs) using Joule-heating
induced electric breakdown. By modulating the heat dissipation along a nanotube, we can confine its thinning and shell breakdown
to occur within localized regions of peak temperatures, which are distributed over one-half of the NT length. The modulation
is achieved by using suitably designed nanomachined heat sinks with different degrees of thermal coupling at different parts
of a current-carrying nanotube. The location of electric breakdown occurs precisely at the regions of high temperatures predicted
by the classical finite-element model of Joule heating in the MWNT. The experiments herein provide new insight into the electric
breakdown mechanism and prove unambiguously that shell removal occurs due to thermal stress, underpinning the diffusive nature
of MWNTs. The method demonstrated here has the potential to be a powerful tool in realizing MWNT bearings with complex architectures
for use in integrated nanoelectromechanical systems (NEMS). In addition, the breakdown current and power in the nanotubes
are significantly higher than those observed in nanotubes without heat removal via additional heat sinks. This indicates future
avenues for enhancing the performance of MWNTs in electrical interconnect and nanoelectronic applications.
PACS 73.63.Fg; 65.80.+n 相似文献
164.
K. Ren Z.Y. Li X. Ren B. Cheng D. Zhang 《Applied Physics A: Materials Science & Processing》2007,87(2):181-185
This paper describes a simple hydrothermal procedure for high-yield synthesis of single-crystalline ZnO hexagonal nanoplates in a surfactant-free system at 70 °C. The structures and morphologies of the synthesized ZnO nanoplates are derived from characterisation by X-ray diffraction, and scanning and transmission electron microscopy. Their optical properties are recorded by Raman and photoluminescence spectroscopy. These ZnO hexagonal nanoplates exhibit the enhanced photocatalytic activity of phenol photodecomposition, suggesting that they could be served as an active system for the treatment of the waste water, in addition to their common applications. PACS 81.10.Dn; 61.10.Nz; 68.37.Hk; 78.55.Hx 相似文献
165.
X.S. Cai 《Applied Surface Science》2006,252(8):2776-2781
SiGe/Si quantum wells (QWs) with different Boron doping concentrations were grown by molecular beam epitaxy (MBE) on p-type Si(1 0 0) substrate. The activation energies of the heavily holes in ground states of QWs, which correspond to the energy differences between the heavy hole ground states and Si valence band, were measured by admittance spectroscopy. It is found that the activation energy in a heavily doped QW increases with doping concentration, which can be understood by the band alignment changes due to the doping in the QWs. Also, it is found that the activation energy in a QW with a doping concentration of 2 × 1020 cm−3 becomes larger after annealing at a temperature of 685 °C, which is attributed to more Boron atoms activation in the QW by annealing. 相似文献
166.
X.L. Zhong 《Applied Surface Science》2006,253(2):417-420
Bi3.25La0.75Ti3O12 (BLT) thin films were fabricated on Pt/Ti/SiO2/Si(1 0 0) substrates by chemical solution deposition (CSD), and the dependence of ferroelectric and dielectric properties of the as-deposited BLT thin films on excess Bi content in precursor sols was studied. It is found that the prepared BLT thin film shows the best polarization-electric field, capacitance-voltage and dielectric constant (?r)-frequency characteristics, when the value of excess Bi content in precursor sols is 10%. In detail, its remnant polarization (2Pr) value is 40 μC/cm2, the capacitance tunability is 21% measured at room temperature under conditions of an applied voltage of 8 V and measurement frequency of 10 kHz, and the ?r is 696 at 100 kHz frequency. 相似文献
167.
Antimonide-based superlattices dedicated to the elaboration of opto-electronic devices have been studied by X-ray scattering techniques. In particular, specular and non-specular X-ray reflectometry experiments have been performed on two MBE-samples elaborated with different shutter sequences at the interfaces. The results have shown a limitation of the incorporation of Sb species in the subsequent InAs layer for one of the samples, as expected.Then, a study on a InGaAs-cap layer/(InGaAs/AlAsSb)N superlattice grown on a InGaAs/InP buffer layer by both specular X-ray reflectometry and High resolution X-ray diffraction is reported. In particular, the results have revealed the presence of a highly disturbed thin-layer on top of the MOVPE-made GaInAs, whose presence has been explained by In-concentration modification during the desoxidation procedure at the surface of the MOVPE-made GaInAs.Beside the results on the Sb-based heterostructures, the use of X-ray scattering metrology as a routinely working non-destructive testing method has been emphasized. 相似文献
168.
W. Ludorf X. Z. Wang D. Bäuerle 《Applied Physics A: Materials Science & Processing》1989,49(2):221-223
Superconducting films of Bi-Sr-Ca-Cu-O on (100) MgO substrates have been fabricated by XeCl-excimer-laser sputtering from ceramic targets of Bi2.5Sr2Ca2Cu3Oy in O2 atmosphere. The films were polycrystalline with the c-axis (30.80±0.02Å) preferably oriented normal to the substrate surface. Without post-annealing the films showed metallic resistance behavior with zero resistance temperatures of up to Tc(0) }- 79 K. The critical current density of the films had values of up to jc(50K)}- 104 A/cm2. 相似文献
169.
A new Co–Fe-based ferromagnetic bulk metallic glass (BMG) was synthesized by copper mould casting method. The thermal stability and crystallization processes were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The soft magnetic behavior was studied by DC magnetic measurements. The high glass formation ability was interpreted in terms of the effective suppression of nucleation and growth of the intermetallic compounds which appear in the multicomponent system during solidification. The high thermal stability indicates that the new Co–Fe-based BMG could be used as high-temperature magnetic material. The low coercivity which was as low as 8 A/m for the as-cast sample was found in the Co–Fe-based metallic glass cylinder with a diameter of 1.5 mm. 相似文献
170.
Two-dimensional antiplane time-harmonic Green’s functions for a circular inhomogeneity with an imperfect interface are derived. Here the linear spring model with vanishing thickness is employed to characterize the imperfect interface. Explicit expressions for the displacement and the stress fields induced by time-harmonic antiplane line forces located both in the unbounded matrix and in the circular inhomogeneity are presented. When the circular frequency approaches zero, our results reduce to those for the static case. Numerical results are presented to show the influence of the frequency and the imperfection of the interface on the stress and displacement fields. 相似文献