全文获取类型
收费全文 | 1862篇 |
免费 | 242篇 |
国内免费 | 121篇 |
专业分类
化学 | 426篇 |
晶体学 | 10篇 |
力学 | 26篇 |
综合类 | 5篇 |
数学 | 27篇 |
物理学 | 1731篇 |
出版年
2024年 | 4篇 |
2023年 | 21篇 |
2022年 | 45篇 |
2021年 | 53篇 |
2020年 | 61篇 |
2019年 | 44篇 |
2018年 | 58篇 |
2017年 | 48篇 |
2016年 | 58篇 |
2015年 | 57篇 |
2014年 | 129篇 |
2013年 | 165篇 |
2012年 | 163篇 |
2011年 | 127篇 |
2010年 | 163篇 |
2009年 | 145篇 |
2008年 | 145篇 |
2007年 | 119篇 |
2006年 | 87篇 |
2005年 | 69篇 |
2004年 | 58篇 |
2003年 | 66篇 |
2002年 | 48篇 |
2001年 | 36篇 |
2000年 | 32篇 |
1999年 | 30篇 |
1998年 | 39篇 |
1997年 | 18篇 |
1996年 | 19篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 14篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 3篇 |
1987年 | 9篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2225条查询结果,搜索用时 26 毫秒
991.
MR Imaging of total hip arthroplasty: comparison among sequences to study the sciatic nerve at 1.5 T
Alberto Tagliafico Andrea Podestà Andrea Assini Leonardo Boccacini Giovanni Serafini Massimo Calabrese Carlo Martinoli 《Magnetic resonance imaging》2010
Purpose
This study was done to test a series of MR sequences for evaluating the sciatic nerve after total hip arthroplasty (THA).Material and Methods
The study protocol was approved by the institutional review board. Informed consent was obtained from all patients. Twenty-five patients (11 men and 14 women mean age: 62.3±5.7 years) with THA were included in this prospective study. MRI protocol included sequences that were preliminarily tailored for nerve imaging in patients with THA: proton density (PD)-weighted turbo SE, T1-weighted turbo SE (TSE) 3 mm thickness, T1-weighted turbo SE (TSE) 6 mm thickness, T1-weighted turbo SE with high bandwidth (TSE hBW), T2- weighted TSE, T2-weighted with fat saturation and short-tau inversion recovery (STIR). For each sequence, we evaluated the visibility of the sciatic nerve using a semiquantitative score (0=total masking; 1=insufficient visibility; 2=sufficient visibility; 3=optimal visibility). The sum of the scores given to each sequence was divided by the maximal sum, obtaining a percentage visibility index. Friedman and sign tests were used for statistical analysis.Results
MR examination time was approximately 40 min. No patients reported pain, heat or symptoms related to nerve stimulation. The visibility index ranged between 88% and 70% for the first four sequences. The T1-weighted TSE hBW sequence had the best visibility index (P<.05). The visibility indexes of the first four sequences were significantly higher (P<.004, sign test) than those of the remaining three sequences.Conclusion
The sciatic nerve could be studied at 1.5 T in patients following THA. The nerve is better visualized with T1-weighted TSE hBW sequences. On T2-weighted sequences and STIR, the visibility of the nerve is low. 相似文献992.
Objective
In general, low-field MRI scanners such as the 0.5- and 1-T ones produce images that are poor in quality. The motivation of this study was to lessen the noise and enhance the signal such that the image quality is improved. Here, we propose a new approach using stochastic resonance (SR)-based transform in Fourier space for the enhancement of magnetic resonance images of brain lesions, by utilizing an optimized level of Gaussian fluctuation that maximizes signal-to-noise ratio (SNR).Materials and Methods
We acquired the T1-weighted MR image of the brain in DICOM format. We processed the original MR image using the proposed SR procedure. We then tested our approach on about 60 patients of different age groups with different lesions, such as arteriovenous malformation, benign lesion and malignant tumor, and illustrated the image enhancement by using just-noticeable difference visually as well as by utilizing the relative enhancement factor quantitatively.Results
Our method can restore the original image from noisy image and optimally enhance the edges or boundaries of the tissues, clarify indistinct structural brain lesions without producing ringing artifacts, as well as delineate the edematous area, active tumor zone, lesion heterogeneity or morphology, and vascular abnormality. The proposed technique improves the enhancement factor better than the conventional techniques like the Wiener- and wavelet-based procedures.Conclusions
The proposed method can readily enhance the image fusing a unique constructive interaction of noise and signal, and enables improved diagnosis over conventional methods. The approach well illustrates the novel potential of using a small amount of Gaussian noise to improve the image quality. 相似文献993.
Barbara Blasiak Boguslaw Tomanek Abedelnasser Abulrob Umar Iqbal Danica Stanimirovic Homam Albaghdadi Tadeusz Foniok Xueqing Lun Peter Forsyth Garnette R. Sutherland 《Magnetic resonance imaging》2010
The aim of the study was to determine the effect of early tumor growth on T2 relaxation times in an experimental glioma model. A 9.4-T magnetic resonance imaging (MRI) system was used for the investigations. An animal model (n=12) of glioma was established using an intracranial inoculation of U87MGdEGFRvIII cells. The imaging studies were performed from Day 10 through Day 13 following tumor inoculation. Tumor blood vessel density was determined using quantitative immunochemistry. Tumor volume was measured daily using MR images. T2 values of the tumor were measured in five areas across the tumor and calculated using a single exponential fitting of the echo train. The measurements on Days 10 and 13 after tumor inoculation showed a 20% increase in T2. The changes in T2 correlated with the size of the tumor. Statistically significant differences in T2 values were observed between the edge of the tumor and the brain tissue on Days 11, 12 and 13 (P=.014, .008, .001, respectively), but not on Day 10 (P=.364). The results show that T2-weighted MRI may not detect glioma during an early phase of growth. T2 increases in growing glioma and varies heterogenously across the tumor. 相似文献
994.
C. Reichhardt C.J. Olson Reichhardt 《Physica C: Superconductivity and its Applications》2010,470(19):722-725
We examine jamming and ratchet effects for vortex matter in superconductors with asymmetric funnel geometries. We show that the vortex–vortex interactions can induce a clogging or jamming effect where it becomes increasingly difficult for the vortices to move through the system. We also find that commensurability effects can arise when certain vortex configurations form highly symmetrical structures in the funnel plaquettes. Due to the asymmetry, the critical currents are different for driving in different directions, leading to a diode effect. We also discuss other possible geometries and approaches that could be used to explore jamming in vortex matter, such as an analog to a granular hopper and a single driven vortex probe moving through an array of other vortices. 相似文献
995.
C.J. Olson Reichhardt C. Reichhardt 《Physica C: Superconductivity and its Applications》2010,470(19):779-781
Recently it has been demonstrated experimentally that it is possible to manipulate an individual vortex in a type-II superconductor using a magnetic force microscope tip. Using numerical simulations, we investigate the dynamics of a single driven vortex in the presence of a periodic pinning array and other vortices. Remarkably, we find that the effective drag on the driven vortex is reduced at the matching fields, which is opposite from the behavior of the critical current when all the vortices are driven. We discuss this effect in the context of the type of dynamics that occur at matching and nonmatching fields. 相似文献
996.
Hideaki Takayanagi R. Inoue T. Akazaki K. Tanaka I. Suemune 《Physica C: Superconductivity and its Applications》2010,470(19):814-817
Superconducting transport is measured between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency. 相似文献
997.
Masaru Kato Yoshiteru Iwamoto Osamu Sato 《Physica C: Superconductivity and its Applications》2010,470(19):747-749
Using the de Gennes–Alexander equation, we have investigated the stable vortex structures in finite superconducting networks (10 × 10 holes) with disordered wires under an external magnetic field. Vortex structures change gradually with increasing magnetic field. For the network with a disordered wire at the edge, vortices are not pinned disordered hole, but enter into the network at the holes with the disorder. But for the network with two disordered wires, the vortex enters at the hole between two disordered wires. This behavior can be considered as the result of the nonlocality of superconductivity. 相似文献
998.
A.D. Thakur S. Ooi S.P. Chockalingam John Jesudasan P. Raychaudhuri K. Hirata 《Physica C: Superconductivity and its Applications》2010,470(20):1112-1114
In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a0, antidot diameter d, lattice symmetry, and orientation) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period and (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore, it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios. 相似文献
999.
R. Tesa? J. Kolá?ek Z. Šimša M. Šindler L. Skrbek K. Il’in M. Siegel 《Physica C: Superconductivity and its Applications》2010,470(19):932-934
Transmission of terahertz waves through a thin layer of the NbN superconductor deposited on a sapphire substrate was studied as a function of temperature in zero field as well as in magnetic field perpendicular to the sample. For photon energies lower than optical gap, detailed temperature measurements in zero field provide BCS-like curves with a pronounced peak below the critical temperature. In accordance with the BCS model, the temperature peak disappears as the energy of incident radiation is increased above the gap. In non-zero field, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at upper critical field. In addition, the presence of quantized vortices in the superconducting film substantially changes shape of the temperature curves. 相似文献
1000.