全文获取类型
收费全文 | 1032篇 |
免费 | 40篇 |
国内免费 | 33篇 |
专业分类
系统科学 | 33篇 |
丛书文集 | 26篇 |
教育与普及 | 22篇 |
理论与方法论 | 4篇 |
现状及发展 | 34篇 |
研究方法 | 13篇 |
综合类 | 973篇 |
出版年
2024年 | 6篇 |
2023年 | 10篇 |
2022年 | 12篇 |
2021年 | 13篇 |
2020年 | 10篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 23篇 |
2014年 | 37篇 |
2013年 | 29篇 |
2012年 | 29篇 |
2011年 | 40篇 |
2010年 | 38篇 |
2009年 | 40篇 |
2008年 | 48篇 |
2007年 | 31篇 |
2006年 | 26篇 |
2005年 | 35篇 |
2004年 | 29篇 |
2003年 | 30篇 |
2002年 | 53篇 |
2001年 | 42篇 |
2000年 | 38篇 |
1999年 | 62篇 |
1998年 | 46篇 |
1997年 | 42篇 |
1996年 | 43篇 |
1995年 | 33篇 |
1994年 | 35篇 |
1993年 | 33篇 |
1992年 | 28篇 |
1991年 | 19篇 |
1990年 | 22篇 |
1989年 | 25篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 4篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1972年 | 1篇 |
1970年 | 1篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有1105条查询结果,搜索用时 796 毫秒
501.
综合国内外套筒圈制工艺的优点,制定了新型套筒自动机的圈制工艺方案,研制了新的凸轮控制系统,使整机质量提高,且明显降噪减振。 相似文献
502.
以枯草杆菌H_(19—7—34)为出发菌株,经紫外线诱变及热处理多次交替处理,获得高产稳定株43—9,该菌株在C/N为1.34,pH6.5~7.0,500ml的三角瓶装量30ml的清液发酵培养基中,用旋转式摇床,转速228r·p·m,37±1℃发酵64hrs,α—淀粉酶平均活性达到526u/ml,最高达550u/ml,比出发菌株提高53.3%,进一步降低了食品用酶的成本,有利于形成大规模工业化生产。 相似文献
503.
Chu Caiyuan Yan Haojing 《东华大学学报(英文版)》1994,(1)
In this paper, two types of high loft nonwoven fabrics have been constructed in the laboratory apparatus made by ourselves with different binders (polyamide and polyethylene powder) and varying binder contents (10%, 20%, 30% and 40%). The tensile and compression properties of these nonwoven fabrics were tested. It was found that one can reduce powder binder content to increase loft and softness of nonwoven fabrics, but it has to sacrifice its tensile strength. Adhesion force between binder and single fibre was also explored. The experiments showed that the adhesion force at the interface between binder and fibre depends on the fibre variety, the fibre surface morphology, heating temperature and heating time etc.. 相似文献
504.
505.
传统骨折疗法是具有中国特点的骨折医疗方式,它经济方便,疗效满意。但也存在某些待解决的问题,约束力的测定就是其中问题之一。本测示仪由传感和数显两部分组成。其用途和特点是:可用于测定约束力和效应力;测量精度可达克的量级;标定、调示容易;具有读数保持功能;便于携带、便于使用;既可用于捆绑好的布带,又使用于捆绑过程中;患者可自行使用。 相似文献
506.
用灰色系统理论中的关联度分析法表征机器磨损状况及油液污染情况变化的动态趋势。关联度法用于铁谱定量技术上,对不同时刻谱片的全谱位曲线或某一区段谱位处曲线进行分析比较,比较曲线间几何形状的接近程度,得到不同的关联度,从而可直观方便地得到机器磨损状况及油液污染情况的变化趋势,显著改善了以往单纯由两点谱位处数据分析机器系统状态产生的大随机误差。 相似文献
507.
Planavsky NJ McGoldrick P Scott CT Li C Reinhard CT Kelly AE Chu X Bekker A Love GD Lyons TW 《Nature》2011,477(7365):448-451
The chemical composition of the ocean changed markedly with the oxidation of the Earth's surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth was characterized by a reducing ocean-atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth's middle age (1.8-1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe(2+)-rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution. 相似文献
508.
Fang Kong ShanJing Mao Kun Du Mian Wu XiaoYan Zhou ChengCai Chu YouPing Wang 《科学通报(英文版)》2011,56(22):2343-2350
Salt stress is one of the major abiotic stresses in agricultural plants worldwide. We used proteomics to analyze the differential
expression of proteins in transgenic OsNAS1 and non-transformant Brassica napus treated with 20 mmol/L Na2CO3. Total protein from the leaves was extracted and separated through a high-resolution and highly repetitive two-dimensional
electrophoresis (2-DE) technology system. Twelve protein spots were reproducibly observed to be upregulated by more than 2-fold
between transgenic and non-transformant B. napus. These 12 spots were digested in-gel with trypsin and characterized by matrix-assisted laser-desorption/ionization time-of-flight
mass spectrometry (MALDI-TOF-MS) to obtain the peptide mass fingerprints. Protein database searching revealed that 5 of these
proteins are involved in salt tolerance: dehydrogenase, glutathione S-transferase, peroxidase, 20S proteasome beta subunit, and ribulose-1,5-bisphosphate carboxylase/oxygenase. The potential
functions of these identified proteins in substance and energy metabolism, stress tolerance, protein degradation, and cell
defense are discussed. The salt tolerance of the transgenic rapeseed was significantly improved by the introduction of the
OsNAS1 gene from Brazilian upland rice of Oryza sativa (cv. IAPAR 9). 相似文献
509.
The hydrogen desorption properties of Li 2 BNH 6 were improved by doping with cobalt. With the addition of CoCl 2 (7 wt%), more than 8 wt% of hydrogen was released from Li 2 BNH 6 at temperatures below 210°C, which is approximately 90°C lower than that of pristine Li 2 BNH 6 . X-ray diffraction, Fourier transform-infrared and Raman characterizations revealed that the dehydrogenation was a stepwise process with the formation of intermediates Li 4 BN 3 H 10 and LiBH 4 and final products of Li 3 BN 2 and LiH. The introduction of Co greatly accelerated the dehydrogenation of Li 4 BN 3 H 10 . X-ray absorption near-edge structure measurements revealed that Co and CoB species formed during ball milling of CoCl 2 with LiBH 4 and LiNH 2 , which may function as catalyst in the subsequent dehydrogenation. 相似文献
510.
Transient performance for output regulation problems of linear discrete-time systems with input saturation is addressed by using the composite nonlinear feedback(CNF) control technique. The regulator is designed to be an additive combination of a linear regulator part and a nonlinear feedback part. The linear regulator part solves the regulation problem independently which produces a quick output response but large oscillations. The nonlinear feedback part with well-tuned parameters is introduced to improve the transient performance by smoothing the oscillatory convergence. It is shown that the introduction of the nonlinear feedback part does not change the solvability conditions of the linear discrete-time output regulation problem. The effectiveness of transient improvement is illustrated by a numeric example. 相似文献