全文获取类型
收费全文 | 976篇 |
免费 | 16篇 |
国内免费 | 15篇 |
专业分类
系统科学 | 21篇 |
丛书文集 | 9篇 |
理论与方法论 | 15篇 |
现状及发展 | 258篇 |
研究方法 | 86篇 |
综合类 | 579篇 |
自然研究 | 39篇 |
出版年
2021年 | 4篇 |
2020年 | 11篇 |
2019年 | 8篇 |
2018年 | 21篇 |
2017年 | 18篇 |
2016年 | 24篇 |
2015年 | 26篇 |
2014年 | 23篇 |
2013年 | 49篇 |
2012年 | 120篇 |
2011年 | 74篇 |
2010年 | 21篇 |
2009年 | 40篇 |
2008年 | 48篇 |
2007年 | 29篇 |
2006年 | 43篇 |
2005年 | 34篇 |
2004年 | 50篇 |
2003年 | 34篇 |
2002年 | 40篇 |
2001年 | 47篇 |
2000年 | 20篇 |
1999年 | 12篇 |
1998年 | 24篇 |
1997年 | 22篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 11篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1974年 | 8篇 |
1973年 | 6篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1970年 | 4篇 |
1969年 | 5篇 |
1968年 | 7篇 |
1967年 | 4篇 |
1966年 | 6篇 |
排序方式: 共有1007条查询结果,搜索用时 31 毫秒
301.
Silicone coatings have been used in this study. The method adopted was the liquid drop analysis on the coated fabrics. The contact angle between a liquid drop and the fabric surface was measured with two liquids continuously and recorded by a computer. The surface energy was calculated by means of Owens method. Kinetic measurement was adopted. The contact angle of liquids on the fabric coated silicone decreased with time was found. A compound solution DX has been found, so that the contact angle of the liquids on the fabric washed with DX becomes constant, and the surface energy of the fabric can be reduced to below 15 mJ/m2. 相似文献
302.
针对界面特性(界面流变特性、界面张力特性、润湿性)在聚合物驱油过程中的重要性,将油藏孔隙介质简化成具有相继收缩和扩张的波纹管模型,根据能量平衡原理建立了波纹管中聚合物溶液驱替时的界面特性参数和驱替速度之间的数学模型,用有限差分方法求解,研究了界面粘度、界面张力、润湿角变化对聚合物/油界面速度的影响。研究结果表明:润湿性变化影响界面的驱替速度,当界面的曲率方向与驱替方向相反时,无因次界面张力和无因次界面粘度的增加都使无因次界面轴向速度降低,且界面轴向驱替速度关于孔隙单元中心不对称。 相似文献
303.
随着油气勘探程度的不断提高,未发现的剩余油气资源越来越少。如何准确预测高勘探程度即成熟探区的剩余油气资源,成为石油投资者和油气资源评价工作者关注的焦点。详细探讨了油藏规模序列法预测油气资源的原理、方法、操作流程及存在的问题,并运用该方法对东营凹陷剩余可探明储量以油气成藏体系为评价单元进行了预测。研究表明,东营凹陷剩余可探明储量约为17.03×108t,仍具有较高的勘探潜力,并且主要集中在东营中央背斜带和东营凹陷北坡两个油气成藏体系内。实践证明,由于油藏规模序列法合理回避了油气成因机理问题,减少了资源评价工作中人为因素的影响,使资源评价结果更为客观。 相似文献
304.
滞洪水库退水闸差异沉降问题的数值模拟分析 总被引:1,自引:0,他引:1
永定河滞洪水库退水闸建成后,在边墩、翼墙后回填土不久,出现了和翼墙外边缘基本平行的贯穿铺盖、消力池和护坦的连续裂缝.为了预测裂缝的发展,结合工程实际条件,用FLAC^3D软件进行了相关的数值模拟。取得了较好的效果. 相似文献
305.
306.
MENEZES Moises Lima de SOUZA Reinaldo Castro PESSANHA Jos Francisco Moreira 《系统科学与复杂性》2014,27(1):29-46
Singular spectrum analysis (SSA) is a technique that decomposes a time series into a set of components, such as, trend, harmonics, and residuals. Leaving out the residual components and adding up the others, the time series can be smoothed. This procedure has been used to model Brazilian electricity consumption and flow series. The PAR(p), periodic autoregressive models, has been broadly used in modelling energy series in Brazil. This paper presents an approach of this decomposition method, by fitting the PAR(p), considering its multivariate version known as multivariate SSA (MSSA). The method was applied to a vector of two wind speed series recorded at two locations in the Brazilian Northeast region. The obtained results, when compared to the univariate decomposition of each series, were far superior, showing that the spatial correlation between the two series were considered by MSSA decomposition stage. 相似文献
307.
308.
Delphine Meffre Julien Grenier Sophie Bernard Françoise Courtin Todor Dudev Ghjuvan’Ghjacumu Shackleford Mehrnaz Jafarian-Tehrani Charbel Massaad 《Cellular and molecular life sciences : CMLS》2014,71(7):1123-1148
Wnt signaling is required for neurogenesis, the fate of neural progenitors, the formation of neuronal circuits during development, neuron positioning and polarization, axon and dendrite development and finally for synaptogenesis. This signaling pathway is also implicated in the generation and differentiation of glial cells. In this review, we describe the mechanisms of action of Wnt signaling pathways and their implication in the development and correct functioning of the nervous system. We also illustrate how a dysregulated Wnt pathway could lead to psychiatric, neurodegenerative and demyelinating pathologies. Lithium, used for the treatment of bipolar disease, inhibits GSK3β, a central enzyme of the Wnt/β-catenin pathway. Thus, lithium could, to some extent, mimic Wnt pathway. We highlight the possible dialogue between lithium therapy and modulation of Wnt pathway in the treatment of the diseases of the nervous system. 相似文献
309.
Solana Abraham Christian Rodríguez Andrea Oviedo Gabriela Murúa 《Journal of Natural History》2018,52(23-24):1493-1508
While expulsion of male ejaculates by the females after copulation has been reported for various animal groups, expulsion followed by consumption of the expelled ejaculate is a rare behaviour outside spermatophylax-producing orthopterans. Among Diptera, this behaviour has been reported for a few species of Piophilidae, Empididae and Ulidiidae. Here we report on its occurrence among Euxesta eluta and Euxesta mazorca (Diptera: Ulidiidae). We also attempt to characterize the mating system of E. eluta in order to facilitate future hypothesis testing to understand the behavioural factors leading to the evolution of this peculiar behaviour. For this, courtship sequences, copulation duration, frequency of ejaculate expulsion and subsequent consumption, and latency to ejaculate expulsion for both E. eluta and E. mazorca were recorded. The time of sexual maturation, the time window of sexual receptivity during the day, and the mating frequency and variance in mating success for males and females (degree of polygamy) were determined for E. eluta. Both E. eluta and E. mazorca males engaged in elaborate courtship sequences involving visual and tactile displays before copulation. Females of both species almost invariably expelled and consumed ejaculates after copulation. Female E. eluta, required a 6- to 9-day period feeding on protein and sugar before becoming sexually receptive. Reproductive activity occurred continuously over the day with an early morning and late afternoon peak. Both males and females could mate multiply, with multiple partners over a 2-h observational period. Sexual network analysis revealed that some males and females had greater mating success than others. Ejaculate consumption appears to be widespread in the genus Euxesta. It is possible that females obtain nutrients from this behaviour while exerting some control over egg fertilization. 相似文献
310.
The following species of the fly genus Lispe are redescribed: L. albitarsis (Stein, 1898), L. latana Snyder, 1949, L. levis (Stein, 1911), L. lisarba Snyder, 1949, L. nasoni (Stein, 1898), L. serotina Wulp, 1896, L. setuligera (Stein, 1911) and L. vilis (Stein, 1911). Additionally, a new diagnosis is proposed for L. rufitibialis Macquart, 1843, and in order to fix the identity of the name the lectotype is designated. To facilitate species identification, a pictorial key for the species of Lispe in the Neotropical region and distribution maps are presented, together with the first published illustrations of the male terminalia of L. latana, L. levis and L. lisarba, and the female of L. lisarba. 相似文献