全文获取类型
收费全文 | 4326篇 |
免费 | 347篇 |
国内免费 | 183篇 |
出版年
2024年 | 11篇 |
2023年 | 68篇 |
2022年 | 95篇 |
2021年 | 101篇 |
2020年 | 125篇 |
2019年 | 155篇 |
2018年 | 145篇 |
2017年 | 117篇 |
2016年 | 126篇 |
2015年 | 149篇 |
2014年 | 262篇 |
2013年 | 388篇 |
2012年 | 155篇 |
2011年 | 178篇 |
2010年 | 169篇 |
2009年 | 213篇 |
2008年 | 202篇 |
2007年 | 196篇 |
2006年 | 209篇 |
2005年 | 200篇 |
2004年 | 140篇 |
2003年 | 153篇 |
2002年 | 127篇 |
2001年 | 106篇 |
2000年 | 58篇 |
1999年 | 61篇 |
1998年 | 102篇 |
1997年 | 78篇 |
1996年 | 69篇 |
1995年 | 51篇 |
1994年 | 60篇 |
1993年 | 73篇 |
1992年 | 37篇 |
1991年 | 31篇 |
1990年 | 21篇 |
1989年 | 26篇 |
1988年 | 41篇 |
1987年 | 24篇 |
1986年 | 32篇 |
1985年 | 39篇 |
1984年 | 45篇 |
1983年 | 26篇 |
1982年 | 52篇 |
1981年 | 46篇 |
1980年 | 24篇 |
1979年 | 19篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1974年 | 9篇 |
排序方式: 共有4856条查询结果,搜索用时 15 毫秒
231.
Brahim Selmaoui Andre Bogdan Andre Auzeby Jacques Lambrozo Yvan Touitou 《Bioelectromagnetics》1996,17(5):364-372
Some epidemiological studies report a relationship between magnetic field exposure and such human diseases as leukemia and immune system disturbances. The few published studies on animals do not demonstrate field exposure-related alterations in hematologic and immune systems. The data presented here are part of a broader study designed to investigate the possible effects of acute exposure to a 50 Hz linearly polarized magnetic field (10 μT) on hematologic and immunologic functions. Thirty-two young men (20–30 years old) were divided into two groups (control group, i.e., sham-exposed, 16 subjects; exposed group, 16 subjects). All subjects participated in two 24 h experiments to evaluate the effects of both continuous and intermittent (1 h “off” and 1 h with the field switched “on” and “off” every 15 s) exposure to linearly polarized magnetic fields. The subjects were exposed to the magnetic field (generated by three Helmholtz coils per bed) from 23:00 to 08:00 while lying down. Blood samples were collected during each session at 3 h intervals from 11:00 to 20:00 and hourly from 22:00 to 08:00. No significant differences were observed between sham-exposed (control) and exposed men for hemoglobin concentration, hematocrit, red blood cells, platelets, total leukocytes, monocytes, lymphocytes, eosinophils, or neutrophils. Immunologic variables [CD3, CD4, CD8, natural killer (NK) cells and B cells] were unaltered. To our knowledge, this study is the first to document the effects of a 50 Hz magnetic field on the circadian rhythm of human hematologic and immune functions, and it suggests that acute exposure to either a continuous or an intermittent 50 Hz linearly polarized magnetic field of 10 μT, at least under the conditions of our experiment, does not affect either these functions or their circadian rhythms in healthy young men. © 1996 Wiley-Liss, Inc. 相似文献
232.
In studies investigating adverse reproductive outcomes associated with video display terminal (VDT) usage, amounts of time spent in front of a VDT or magnetic field (MF) levels in front of the VDT are used as surrogate measures of subject's MF exposure. However, the relevance of such surrogates to actual exposures has not been demonstrated, and the validity of the use of such measures as a surrogate for the actual MF exposure is only speculative. This study examines 1) measurements of MFs at frequencies of approximately 30–1000 Hz at a fixed distance from the VDTs, 2) reported hours of VDT use, and 3) reported distance between the VDT and the subject's waist as surrogate measures for the average MF exposure level of a VDT user during one 8 h workday. The results showed a weak correlation between the average exposure level of a VDT user and the MF 46 cm from a VDT (R = 0.52, n = 67, P < 0.001). This study showed no association between self-reported hours of VDT usage, or self-reported distance between waist and VDT, and the average MF exposures. Moreover, individuals' average MF exposures did not seem to be affected by other variables, such as position of a VDT on the desk, hours of desk use, and the VDT type (color vs. monochrome). These findings indicate that VDT exposures within office settings are complex and cannot be easily predicted by surrogates. © 1996 Wiley-Liss, Inc. 相似文献
233.
It has been suggested that “double-wound” (bifilar) exposure coils are capable of producing a sham environment in which hum and vibration will be “similar” to the field-exposed condition. We found by direct measurements in a bifilar coil system that vibration amplitude in sham and exposed conditions differed by a factor of 50 when our test system was driven at B = 10 mT. We also found that the normal laboratory environment can include vibrations of an intensity similar to that produced by the exposure system, although not necessarily of similar spectral distribution. © 1996 Wiley-Liss, Inc. 相似文献
234.
235.
Piezoelectric Pump Used in Bionic Underwater Propulsion Unit 总被引:1,自引:0,他引:1
A new piezoelectric pump can pump liquid either forward or backward and adjust the flow rate. Thus an object can be driven forward or backward at different speeds. The driver of the pump, a circular piezoelectric plate, is modelled by Finite Element Method (FEM) in ANSYS and its performance is simulated and analyzed. The pump gives the best performance when the driving signals of the inlet and outlet valves have a bigger duty cycle and the plate has a higher voltage applied. 相似文献
236.
《Structure (London, England : 1993)》2020,28(3):336-347.e4
- Download : Download high-res image (195KB)
- Download : Download full-size image
237.
Evgeny B. Lyskov Jukka Juutilainen Veikko Jousmki Juhani Partanen Sviatoslav Medvedev Osmo Hnninen 《Bioelectromagnetics》1993,14(2):87-95
The influence of sinusoidal 45-Hz magnetic fields on the brain functions of 20 volunteers was investigated in a double-blind study using spectral analysis of EEG and measurements of Omega potentials and reaction time (RT). The field strength was 1,000 A/m (1.26 mT) and the duration of exposure was 1 h. Ten volunteers were exposed to a continuous field and ten received an intermittent exposure (1 s on/1 s off). Each person received one real and one sham exposure. One half of the volunteers got the real exposure first and the sham treatment after at least 24 h. For the rest, the sequence was inverse. The measurements of EEG, omega potentials and RT were performed before and after each exposure. Several statistically significant changes were observed, most of them after intermittent exposure. In the EEG, an increase of alpha (7.6–13.9 Hz) activity and a decrease of delta (1.5–3.9 Hz) activity were observed. β waves (14.2–20 Hz) increased in the frontal derivations as did the total power in occipital derivations. The mean and peak frequencies of EEG increased mainly in the frontal derivations. No direct effects on RT were seen. Learning to perform the RT test (decrease of RT in repeated trials), however, seemed to be affected by the exposure. The persons who received real exposure first learned more slowly than those who got sham exposure first. Further experiments are necessary to confirm the findings and for understanding the mechanisms of the effects. © 1993 Wiley-Liss. Inc. 相似文献
238.
K E McCloskey K Comella J J Chalmers S Margel M Zborowski 《Biotechnology and bioengineering》2001,75(6):642-655
Magnetic cell separation methods commonly utilize paramagnetic materials conjugated to antibodies that target specific cell surface molecules. The amount of magnetic material bound to a cell is directly proportional to the magnetophoretic mobility of that cell. A mathematical model has been developed which characterizes the fundamental parameters controlling the amount of magnetic material bound, and thus, the magnetophoretic mobility of an immunomagnetically labeled cell. In characterization of the paramagnetic labeling, one of the parameters of interest is the increase in magnetophoretic mobility due to the secondary antibody binding to multiple epitopes on the primary antibody, referred to as the "secondary antibody binding amplification," Psi. Secondary antibody-binding amplification has been investigated and quantitated by comparing the mobilities of lymphocytes directly labeled with anti-CD4 MACS (Miltenyi Biotec, Auburn, CA) magnetic nanoparticle antibody with the mobilities of lymphocytes from the same sample labeled with two different indirect antibody-labeling schemes. Each indirect labeling scheme incorporated a primary mouse anti-CD4 FITC antibody that provides both FITC and mouse-specific binding sites for two different secondary antibody-magnetic nanoparticle conjugates: either anti-FITC MACS magnetic nanoparticle antibody or anti-mouse MACS magnetic nanoparticle antibody. The magnetophoretic mobilities of the immunomagnetically labeled cells were obtained using Cell Tracking Velocimetry (CTV). The results indicate that an average of 3.4 anti-FITC MACS magnetic nanoparticle antibodies bind to each primary CD4 FITC antibody, Psi(1,2f) = 3.4 +/- 0.33, and that approximately one, Psi(1,2m) = 0.98 +/- 0.081, anti-mouse MACS magnetic nanoparticle antibody binds to each primary mouse CD4 FITC antibody on a CD4 positive lymphocyte. These results have provided a better understanding of the antibody-binding mechanisms used in paramagnetic cell labeling for magnetic cell separation. 相似文献
239.
One critical biophysical feature of environmental-level magnetic field (MF) interactions with biological systems is the time-scale
of interaction. A recently proposed fast/slow hypothesis states that a fast mechanism can only sense the instantaneous absolute
value of the MF, and that a slow mechanism is potentially capable of sensing features such as frequency and relative orientation
and magnitude of the field components. Here we applied the fast/slow hypothesis to a breast cancer model system: A 1.2 μT(rms),
60-Hz field inhibits tamoxifen’s (TAM’s) cytostatic action in MCF-7 cells via a MF interaction. We measured the growth of
MCF-7 cells treated with TAM over 7 d, within different MFs: a sinusoidal, 60-Hz, 0.2-μT(rms) field; a sinusoidal, 60-Hz,
1.2-μT(rms) field; and a full-wave rectified version of the 1.2-μT(rms) sinusoidal field. A fast mechanism should not be able
to distinguish between the latter two exposures. We observe that the rectified 1.2-μT field does not inhibit TAM’s action,
but that the 1.2-μT sinusoidal field does. Therefore, the 1.2-μT MF inhibition of TAM’s cytostatic action operates via a relatively
slow mechanism, and we predict that there exists a biologically dynamic complex capable of sensing a 1.2-μT, 60-Hz sinusoidal
MF with an intrinsic time-scale of 17 ms or longer, the period of the 60-Hz applied field. 相似文献
240.
The effects of different non-bonded parameters of force fields for NMR structure calculation on the quality of the resulting NMR solution structures were investigated using Interleukin 4 as a model system. NMR structure ensembles were calculated with an ab initio protocol using torsion angle dynamics. The calculations were repeated with five different non-bonded energy functions and parameters. The resulting ensembles were compared with the available X-ray structures, and their quality was assessed with common structure validation programs. In addition, the impact of torsion angle restraints and dihedral energy terms for the sidechains and the backbone was studied. The further improvement of the quality by refinement in explicit solvent was demonstrated. The optimal parameters, including those necessary for water refinement, are available in the new version of the PARALLHDG force field. 相似文献