全文获取类型
收费全文 | 600篇 |
免费 | 31篇 |
专业分类
电工技术 | 7篇 |
化学工业 | 77篇 |
金属工艺 | 8篇 |
机械仪表 | 9篇 |
建筑科学 | 35篇 |
能源动力 | 14篇 |
轻工业 | 73篇 |
水利工程 | 12篇 |
石油天然气 | 1篇 |
无线电 | 57篇 |
一般工业技术 | 96篇 |
冶金工业 | 180篇 |
原子能技术 | 8篇 |
自动化技术 | 54篇 |
出版年
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 11篇 |
2020年 | 7篇 |
2019年 | 6篇 |
2018年 | 14篇 |
2017年 | 8篇 |
2016年 | 10篇 |
2015年 | 14篇 |
2014年 | 19篇 |
2013年 | 23篇 |
2012年 | 35篇 |
2011年 | 34篇 |
2010年 | 29篇 |
2009年 | 15篇 |
2008年 | 31篇 |
2007年 | 25篇 |
2006年 | 16篇 |
2005年 | 15篇 |
2004年 | 23篇 |
2003年 | 15篇 |
2002年 | 10篇 |
2001年 | 6篇 |
2000年 | 6篇 |
1999年 | 9篇 |
1998年 | 36篇 |
1997年 | 27篇 |
1996年 | 16篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 9篇 |
1974年 | 5篇 |
1972年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有631条查询结果,搜索用时 15 毫秒
81.
82.
The propagation of a transient electromagnetic field through a cold, isotropic, lossless, inhomogeneous plasma is considered. The electron density of the plasma decreases exponentially in the direction of propagation. For incident transient signals whose Laplace transforms are regular at infinity and whose only singularities are poles, the transient electric field propagated through the plasma can be expressed as a sum of exponential waves at discrete complex frequencies. The characteristic frequencies of the inhomogeneous plasma are related to the zeros of the modified Bessel function of the first kind of fixed argument and variable order. When the Laplace transform of the incident signal is not well-behaved at infinity, approximate methods may be used to evaluate the transient electric field. The propagation of a step sine wave and of a quasi-monochromatic Gaussian pulse are considered. It is found that for plasmas characteristic of reentry boundary layers, the effect of the plasma layer on dispersion is very small. The principal effect of the layer is to attenuate a signal propagated through it. 相似文献
83.
Earlier studies have shown that some extracts of nonpolar lipids from lemon shark liver are able to stimulate the reticuloendothelial
system (RES) of experimental animals. This communication reports the identification of the coenzymes Q as the constituents
of a fraction from lemon shark liver lipid which showed intense biological activity. 相似文献
84.
The specific activity of enkephalinase in endometrial tissue of nonpregnant ovulatory women is correlated in a highly significant, positive manner with the plasma level of progesterone. The specific activity and levels of enkephalinase messenger ribonucleic acid and immunoreactive protein also are increased in human endometrial stromal cells in culture by treatment with a synthetic progestin, medroxyprogesterone acetate (MPA), in a time- and dose-dependent manner. From an analysis of the temporal relationship between the specific activity and half-life of enkephalinase in endometrial tissue and the level of progesterone in plasma, it appeared highly likely that some mechanism, in addition to progesterone withdrawal, was operative to reduce enkephalinase activity in endometrium during the late luteal phase of the ovarian cycle before progesterone levels had declined below those known to be effective for progesterone action. In stromal cells previously (and concurrently) treated with MPA (10(-9) mol/L), the addition of transforming growth factor-beta 1 (TGF beta 1) or TGF beta 2 (1 ng/mL) to the medium caused a decrease in enkephalinase specific activity despite the continued presence of MPA. The half-life of enkephalinase (activity) in stromal cells treated with MPA plus TGF beta 1 was 2.8 days, which is similar to the computed half-life for enkephalinase in endometrial tissue during the mid- to late secretory phase of the endometrial cycle (2.5 days). Simultaneous treatment of endometrial stromal cells with MPA (10(-9) mol/L) and TGF beta 1 (1 ng/ mL) prevented the progestin-induced increase in enkephalinase specific activity and immunoreactive enkephalinase protein. Thus, TGF beta acts to oppose the progesterone-induced increase in enkephalinase expression in endometrial stromal cells, even in the continued presence of MPA. 相似文献
85.
86.
E. Miranda E. O’Connor G. Hughes P. Casey K. Cherkaoui S. Monaghan R. Long D. O’Connell P.K. Hurley 《Microelectronic Engineering》2009,86(7-9):1715-1717
The degradation dynamics of magnesium oxide (MgO) layers in MOS structures has been investigated. It is shown that the shift of the conduction characteristics caused by both ramped and constant voltage stresses is well described by power-law models and that the signature of charge trapping is still visible even after the dielectric breakdown (BD) of the insulating film. The occurrence of progressive BD in 20-nm thick films is also reported. 相似文献
87.
This paper investigates data compression methods for time-of-flight (TOF) positron emission tomography (PET), which rebin the 3-D TOF measurements into a set of 2-D TOF data for a stack of transaxial slices. The goal of this work is to develop rebinning algorithms that are more accurate than the TOF single-slice-rebinning (TOF-SSRB) method proposed by Mullani in 1982. Two approaches are explored. The first one is based on a partial differential equation, which expresses a consistency condition for TOF-PET data with a Gaussian TOF profile. From this equation we derive an analytical rebinning algorithm, which is unbiased in the limit of continuous sampling. The second approach is discrete: each 2-D rebinned data sample is calculated as a linear combination of the 3-D TOF samples in the same axial plane parallel to the axis of the scanner. The coefficients of the linear combination are precomputed by optimizing a cost function which enforces both accuracy and good variance reduction, models the TOF profile, the axial PSF of the LORs, and the specific sampling scheme of the scanner. Measurements of a thorax phantom on a prototype TOF-PET scanner with a resolution of 550 ps show that the proposed discrete method improves the bias-variance trade-off and is a promising alternative to TOF-SSRB when data compression is required to achieve clinically acceptable reconstruction time. 相似文献
88.
Fleming R.J. Ammala A. Lang S.B. Casey P.S. 《Dielectrics and Electrical Insulation, IEEE Transactions on》2008,15(1):118-126
DC conductivity and ac impedance measurements were made in air and in vacuum on samples of low density polyethylene to which nano-sized and micro-sized ZnO particles and a dispersant had been added. The samples were 150-200 mum thick. The temperature range was 30-70degC. The temperature dependence of the vacuum dc conductivity in samples containing the dispersant and 10% w/w nanosized ZnO followed an Arrhenius relationship closely, the conductivity being 1-2 orders of magnitude lower than that of a sample containing dispersant only. The addition of 10% w/w microsized ZnO had very little effect on the dc conductivity. The ac measurements were made in the frequency range 10 mHz-1 MHz. Addition of nanoparticles increased the ac conductivity at higher frequencies but decreased it at lower frequencies, the cross-over frequency increasing with increasing temperature. The real part of the relative permittivity of samples with nanoparticles was increased relative to that of samples containing dispersant only, at all temperatures, but the corresponding values in samples with microparticles were unchanged, within experimental error. Space charge profiles were obtained using the laser-intensity-modulation-method (LIMM). Space charge densities of order 300 Cm-3 were measured in the bulk near the electrodes, several hours after poling at field strengths around 30 kV/mm. 相似文献
89.
Sijie Wang Dr. Aktan Alpsoy Surbhi Sood Sandra Carolina Ordonez-Rubiano Dr. Alisha Dhiman Yixing Sun Guanming Jiao Dr. Casey J. Krusemark Dr. Emily C. Dykhuizen 《Chembiochem : a European journal of chemical biology》2021,22(13):2335-2344
Polycomb group (PcG) proteins are epigenetic regulators that facilitate both embryonic development and cancer progression. PcG proteins form Polycomb repressive complexes 1 and 2 (PRC1 and PRC2). PRC2 trimethylates histone H3 lysine 27 (H3K27me3), a histone mark recognized by the N-terminal chromodomain (ChD) of the CBX subunit of canonical PRC1. There are five PcG CBX paralogs in humans. CBX2 in particular is upregulated in a variety of cancers, particularly in advanced prostate cancers. Using CBX2 inhibitors to understand and target CBX2 in prostate cancer is highly desirable; however, high structural similarity among the CBX ChDs has been challenging for developing selective CBX ChD inhibitors. Here, we utilize selections of focused DNA encoded libraries (DELs) for the discovery of a selective CBX2 chromodomain probe, SW2_152F. SW2_152F binds to CBX2 ChD with a Kd of 80 nM and displays 24-1000-fold selectivity for CBX2 ChD over other CBX paralogs in vitro. SW2_152F is cell permeable, selectively inhibits CBX2 chromatin binding in cells, and blocks neuroendocrine differentiation of prostate cancer cell lines in response to androgen deprivation. 相似文献
90.
Casey Jessop Johan Ahlstrm Christer Persson Yubin Zhang 《Fatigue & Fracture of Engineering Materials & Structures》2020,43(1):201-208
Rolling contact fatigue cracks and thermally induced defects are common problems in the railway industry especially as demands for increasing loads, speeds, and safety continue to rise. Often, the two types of defects are found together in the field, however, whether one causes the other to occur is not completely agreed upon. The effect of thermal damage, in the form of a martensite spot on pearlitic steel test bars, on the fatigue life in uniaxial low cycle fatigue experiments was investigated by the authors. However, the focus of the current work was to characterize the damage evolution from the low cycle fatigue (LCF) tests and correlate the crack initiation and propagation with the initial thermal damage. Residual stress measurements, digital image correlation, and X‐ray tomography were used to characterize the effects of the thermal damage before, during, and after fatigue testing, respectively. It was found that the thermal damage causes strain accumulation and crack initiation at the interface between the two materials. The strain evolution was visualized using digital image correlation (DIC), clearly showing the strain concentrations at the top and bottom of the white etching layers (WEL), where the residual stresses are also most tensile. X‐ray tomography confirmed the planar crack growth from the martensite spot. 相似文献