全文获取类型
收费全文 | 1361篇 |
免费 | 47篇 |
国内免费 | 8篇 |
专业分类
化学 | 824篇 |
晶体学 | 3篇 |
力学 | 13篇 |
数学 | 216篇 |
物理学 | 360篇 |
出版年
2023年 | 10篇 |
2022年 | 13篇 |
2021年 | 14篇 |
2020年 | 29篇 |
2019年 | 31篇 |
2018年 | 19篇 |
2017年 | 21篇 |
2016年 | 27篇 |
2015年 | 32篇 |
2014年 | 27篇 |
2013年 | 69篇 |
2012年 | 77篇 |
2011年 | 78篇 |
2010年 | 37篇 |
2009年 | 33篇 |
2008年 | 73篇 |
2007年 | 71篇 |
2006年 | 59篇 |
2005年 | 52篇 |
2004年 | 56篇 |
2003年 | 42篇 |
2002年 | 45篇 |
2001年 | 15篇 |
2000年 | 29篇 |
1999年 | 21篇 |
1998年 | 8篇 |
1997年 | 23篇 |
1996年 | 12篇 |
1995年 | 14篇 |
1994年 | 16篇 |
1993年 | 20篇 |
1992年 | 16篇 |
1991年 | 21篇 |
1990年 | 25篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 18篇 |
1979年 | 8篇 |
1978年 | 17篇 |
1977年 | 18篇 |
1976年 | 14篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 12篇 |
排序方式: 共有1416条查询结果,搜索用时 31 毫秒
111.
Y. Takase J. I. Scheinbeim B. A. Newman 《Journal of Polymer Science.Polymer Physics》1989,27(11):2347-2359
The effects of annealing temperatures on stretched poly(vinylidene fluoride) film were systematically studied from the “as-received” condition up to the melting range (180°C). X-ray diffraction studies indicate that the annealing process brings about better chain packing and increased crystallite perfection. The elastic modulus and piezoelectric strain and stress constants, d3, and e31, decrease as the annealing temperature Ta increases up to 160°C, while the remanent polarization Pr remains almost constant. Some of these characteritics may be interpreted in terms of a morphological transformation of microfibrils. The values of Pr, d31, and e31 increase dramatically as Ta increases from 160 to 180°C; Pr increases from 56 to 85 mC/m2, d31 from 20.2 to 27.7 pC/N, and e31 from 51.4 to 65.2 mC/m2. As a result, the values of Pr and d31 were the largest recorded from any of the samples used in the present study. e31 showed a value close to the largest one; this usually occurs in unannealed samples. Samples annealed in the melting range also exhibit significantly improved ageing characteristics. The large value of Pr and the small relaxation strength of both the dielectric constant and elastic modulus indicates that the largest crystallinity obtained is approximately 70%. 相似文献
112.
A nontrivial function having the properties described in the title is given explicitly. 相似文献
113.
114.
Carlos Kozameh Ezra T. Newman Gilberto Silva-Ortigoza 《General Relativity and Gravitation》2008,40(10):2043-2050
A problem in general relativity is how to extract physical information from solutions to the Einstein equations. Most often
information is found from special conditions, e.g., special vector fields, symmetries or approximate symmetries. Our concern
is with asymptotically flat space–times with approximate symmetry: the BMS group. For these spaces the Bondi four-momentum
vector and its evolution, found at infinity, describes the total energy–momentum and the energy–momentum radiated. By generalizing
the simple idea of the transformation of (electromagnetic) dipoles under a translation, we define (analogous to center of
charge) the center of mass for asymptotically flat Einstein–Maxwell fields. This gives kinematical meaning to the Bondi four-momentum,
i.e., the four-momentum and its evolution which is described in terms of a center of mass position vector, its velocity and
spin-vector. From dynamical arguments, a unique (for our approximation) total angular momentum and evolution equation in the
form of a conservation law is found.
Third Award in the 2008 Essay Competition of the Gravity Research Foundation. 相似文献
115.
Giant-cell tumor of the tibia in a child presenting as an expansile metaphyseal lesion with fluid-fluid levels on MR 总被引:1,自引:0,他引:1
Approximately two percent of giant-cell tumors (GCT) occur in the skeletally immature. The patient presented is four years old with a cystic giant-cell tumor of the tibial metaphysis. Fluid-fluid levels are demonstrated on CT and MR and likely represent the underlying pathophysiology. 相似文献
116.
Silvin P. Knight Louise Newman John D. OConnor James Davis Rose Anne Kenny Roman Romero-Ortuno 《Entropy (Basel, Switzerland)》2021,23(1)
In this cross-sectional study, the relationship between noninvasively measured neurocardiovascular signal entropy and physical frailty was explored in a sample of community-dwelling older adults from The Irish Longitudinal Study on Ageing (TILDA). The hypothesis under investigation was that dysfunction in the neurovascular and cardiovascular systems, as quantified by short-length signal complexity during a lying-to-stand test (active stand), could provide a marker for frailty. Frailty status (i.e., “non-frail”, “pre-frail”, and “frail”) was based on Fried’s criteria (i.e., exhaustion, unexplained weight loss, weakness, slowness, and low physical activity). Approximate entropy (ApEn) and sample entropy (SampEn) were calculated during resting (lying down), active standing, and recovery phases. There was continuously measured blood pressure/heart rate data from 2645 individuals (53.0% female) and frontal lobe tissue oxygenation data from 2225 participants (52.3% female); both samples had a mean (SD) age of 64.3 (7.7) years. Results revealed statistically significant associations between neurocardiovascular signal entropy and frailty status. Entropy differences between non-frail and pre-frail/frail were greater during resting state compared with standing and recovery phases. Compared with ApEn, SampEn seemed to have better discriminating power between non-frail and pre-frail/frail individuals. The quantification of entropy in short length neurocardiovascular signals could provide a clinically useful marker of the multiple physiological dysregulations that underlie physical frailty. 相似文献
117.
Krause KM Vick DW Malac M Brett MJ 《Langmuir : the ACS journal of surfaces and colloids》2010,26(22):17558-17567
The high surface area, large aspect ratio, and porous nature of nanorod arrays make them excellent foundation materials for many devices. Of the many synthesis techniques for forming nanorods, glancing angle deposition (GLAD) offers one of the more straightforward and flexible methods for ensuring control of alignment, porosity, and architecture of the nanorods. Here we demonstrate the first use of a dual-beam (focused ion beam (FIB) combined with scanning electron microscopy (SEM)) instrument to section and image the internal morphology of a nanorod array fabricated using the GLAD technique. We have used the FIB-SEM to reconstruct the 3D composition of TiO(2) nanorods, allowing us to visualize for the first time the core structures of many potential devices. We have also been able to probe the relationship between critical parameters such as diameter (w(act)), internanorod spacing (ν(act)), center-to-center spacing (c(act)), and nanorod population density (d(act)) and the depth of the nanocolumn (t) for a single homogeneous structure. A continuous data set was obtained from a single 5-μm-thick GLAD film, avoiding the artifacts arising from the analysis of the top surfaces of multiple samples of varying thicknesses. An analysis of the acquired sectioned data has allowed us to determine that the critical nanocolumn parameters follow a power-law scaling trend with w(act) = 9.4t(0.35) nm, ν(act) = 15.2t(0.25) nm, c(act) = 24.8t(0.31) nm, and d(act) = 3402t(-0.65) columns μm(-2). Using the FIB/SEM images acquired for the TiO(2) nanorods, we have also investigated the evolution of individual nanocolumns and have observed that bifurcation and branching play a significant role in the extinction or survival of these nanorods. These findings will allow for the optimization of nanorod properties for device applications. Also, the FIB sectioning and reconstruction process developed here will permit for the investigation of nanorod arrays formed from a range of synthesis techniques and materials. 相似文献
118.
A new sensor has been developed for the simultaneous detection of cadmium, lead, copper and mercury, using differential pulse and square wave anodic stripping voltammetry (DPASV and SWASV) at a graphite–polyurethane composite electrode with SBA‐15 silica organofunctionalized with 2‐benzothiazolethiol as bulk modifier. The heavy metal ions were preconcentrated on the surface of the modified electrode at ?1.1 V vs. SCE where they complex with 2‐benzothiazolethiol and are reduced to the metals, and are then reoxidized. Optimum SWASV conditions lead to nanomolar detection limits and simultaneous determination of Cd2+, Pb2+, Cu2+ and Hg2+ in natural waters was achieved. 相似文献
119.
Sidney Xavier dos Santos Éder T. G. Cavalheiro Christopher M. A. Brett 《Electroanalysis》2010,22(23):2776-2783
A new composite electrode based on multiwall carbon nanotubes (MWCNT) and silicone‐rubber (SR) was developed and applied to the determination of propranolol in pharmaceutical formulations. The effect of using MWCNT/graphite mixtures in different proportions was also investigated. Cyclic voltammetry and electrochemical impedance spectroscopy were used for electrochemical characterization of different electrode compositions. Propranolol was determined using MWCNT/SR 70 % (m/m) electrodes with linear dynamic ranges up to 7.0 µmol L?1 by differential pulse and up to 5.4 µmol L?1 by square wave voltammetry, with LODs of 0.12 and 0.078 µmol L?1, respectively. Analysis of commercial samples agreed with that obtained by the official spectrophotometric method. The electrode is mechanically robust and presented reproducible results and a long useful life. 相似文献
120.
Fernando C. Moraes Lucia H. Mascaro Sergio A. S. Machado Christopher M. A. Brett 《Electroanalysis》2010,22(14):1586-1591
A copper phthalocyanine/multiwalled carbon nanotube film‐modified glassy carbon electrode has been used for the determination of the herbicide glyphosate (Gly) at ?50 mV vs. SCE by electrochemical oxidation using differential pulse voltammetry (DPV). Cyclic voltammetry and electrochemical impedance spectroscopy showed that Gly is adsorbed on the metallic centre of the copper phthalocyanine molecule, with formation of Gly‐copper ion complexes. An analytical method was developed using DPV in pH 7.4 phosphate buffer solution, without any pretreatment steps: Gly was determined in the concentration range of 0.83–9.90 μmol L?1, with detection limit 12.2 nmol L?1 (2.02 μg L?1). 相似文献