全文获取类型
收费全文 | 4579篇 |
免费 | 525篇 |
国内免费 | 235篇 |
专业分类
化学 | 393篇 |
晶体学 | 3篇 |
力学 | 299篇 |
综合类 | 117篇 |
数学 | 3417篇 |
物理学 | 1110篇 |
出版年
2023年 | 47篇 |
2022年 | 102篇 |
2021年 | 123篇 |
2020年 | 96篇 |
2019年 | 98篇 |
2018年 | 83篇 |
2017年 | 159篇 |
2016年 | 184篇 |
2015年 | 108篇 |
2014年 | 263篇 |
2013年 | 402篇 |
2012年 | 216篇 |
2011年 | 242篇 |
2010年 | 219篇 |
2009年 | 277篇 |
2008年 | 285篇 |
2007年 | 264篇 |
2006年 | 223篇 |
2005年 | 228篇 |
2004年 | 162篇 |
2003年 | 136篇 |
2002年 | 138篇 |
2001年 | 138篇 |
2000年 | 133篇 |
1999年 | 114篇 |
1998年 | 115篇 |
1997年 | 85篇 |
1996年 | 79篇 |
1995年 | 68篇 |
1994年 | 62篇 |
1993年 | 51篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 48篇 |
1989年 | 28篇 |
1988年 | 21篇 |
1987年 | 39篇 |
1986年 | 21篇 |
1985年 | 51篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 20篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 19篇 |
1978年 | 14篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1974年 | 6篇 |
1973年 | 7篇 |
排序方式: 共有5339条查询结果,搜索用时 62 毫秒
161.
This paper presents the theoretical analysis of adaptive multiuser RAKE receiver scheme in frequency selective fading channel for direct-sequence code division multiple access (DS-CDMA) system. Least mean square (LMS) algorithm is used to estimate the channel coefficients. Chaotic sequences are used as spreading sequence and corresponding bit error rate (BER) in closed form is derived for imperfect channel estimation conditions. Performances of chaotic sequences are compared with pseudorandom noise (PN) sequences. Under perfect synchronization assumption, various simulation results are shown to investigate the performance of the proposed system. 相似文献
162.
163.
Nylon 6 and 6,6 literature data are collected over a wide range of water concentrations and temperatures (0 ≤ [W]0 ≤ 40.8 wt%, 200 ≤ T ≤300 °C) and used to fit parameters in an updated batch reactor model. The resulting copolymerization model uses side reactions to account for the complex influence of water on kinetics and reaction equilibria. The proposed parameter estimates result in a significant improvement in the fit to the data, corresponding to a 73% reduction in the weighted‐least‐squares objective function compared to when the parameters of Arai et al. are used. Copolymerization simulations are conducted at industrially relevant conditions, shedding light on the complex influence of water and on the potential to include waste nylon 6 cyclic dimer in the feedstock. The model and parameter estimates will be helpful in future models of nylon 6/6,6 copolymerization in continuous reactor systems. 相似文献
164.
This investigation is dedicated to unlocking the hidden potential of discarded cosmetics towards building green sustainable road pavements in the future. It is particularly aiming at exploring waste lipstick (WLS) as a high-quality functional additive for advanced asphalt mix technologies. To fuel this novel innovation, the effect of various WLS doses (e.g., 5, 10, and 15 wt.%) on the performance of base AP-5 asphalt cement was studied in detail. A wide array of cutting-edge analytical lab techniques was employed to inspect in-depth the physicochemical, microstructural, thermo-morphological, and rheological properties of resultant admixtures including: elemental analysis, Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thin-layer chromatography-flame ionization detection (TLC-FID), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), needle penetration, ring and ball softening point, Brookfield viscometer, ductility, and dynamic shear rheometer (DSR) tests. Unlike the unstable response of asphaltenes, the additive/artificial aging treatments increased the fraction of resins the most, and decreased that of aromatics; however, asphaltenes did not impair the saturates portion, according to Iatroscan research. FT-IR scan divulged that the WLS-asphalt interaction was physical rather than chemical. XRD diagnosis not only revealed an obvious correlation between the asphaltenes content and the fresh-binder crystallinity but also revealed the presence of fillers in the WLS, which may generate outstanding technical qualities to bituminous mixes. According to AFM/SEM analyses, the stepwise incorporation of WLS grew the magnitude of the “bee-shaped” microstructures and extended the roughness rate of unaged/aged binders. The prolonged consumption of the high thermal-stable additive caused a remarkable drop in the onset degradation and glass transition temperature of mixtures, thus enhancing their workability and low-temperature performance, according to TGA/DTGA/DSC data. The DSR and empirical rheological experiments demonstrated that the WLS could effectively lower the manufacturing and compaction temperatures of asphalt mixes and impart them with valuable anti-aging/fatigue-cracking assets. In a nutshell, the use of waste lipstick as an asphalt modifier is viable and cost-effective and could attenuate the pollution arisen from the beauty sector, while improving the performance of hot/warm asphalt mixes (HAM/WAM) and extending the service life of roadways. 相似文献
165.
对全国农残水平测试中毒死蜱、氯氰菊酯数据进行统计分析,在数据统计分布特征研究基础上,使用内核密度估计进行数据多态性分析,使用bootstrap模拟取样法对数据样本值重复取样,以获得稳健的水平测试样品待测物含量代表值估计、标准误差及置信区间描述,证明以bootstrap模拟取样法获取的均值与标准偏差作为有限单次样本代表值是合理、有效的,解决了四分位稳健统计方法对非正态多态分布代表值估计不稳定问题及取样理论中取样样本数限制的瓶颈,为能力验证计划指定值的获取提供了一种新方法。 相似文献
166.
167.
一种四次有理插值样条及其逼近性质 总被引:3,自引:0,他引:3
1引言有理样条函数是多项式样条函数的一种自然推广,但由于有理样条空间的复杂性,所以有关它的研究成果不象多项式样条那样完美,许多问题还值得进一步的研究.近几十年来,有理插值样条,特别是有理三次有理插值样条,由于它们在曲线曲面设计中的应用,已有许多学者进行了深入研究,取得了一系列的成果(见[1]-[7]).但四次有理插值样条由于其构造所花费的计算量太大以及在使用上很不方便而让人们忽视了其重要的应用价值,因此很少有人研究他们.实际上,在某些情况下四次有理插值样条有其独特的应用效果,如文[8]建立的一种具有局部插值性质的分母为二次的四次有理样条,即一个剖分 相似文献
168.
169.
170.
单指标面板模型已广泛应用于各学科领域的研究中,其估计方法较为丰富,然而鲜有估计方法将个体内的相关性考虑在内.基于此,本文研究了一类个体内存在相关性的固定效应部分线性单指标面板模型,采用惩罚二次推断函数法和LSDV法相结合的方法对模型进行估计,证明了所得估计量的一致性和渐近正态性.Monte Carlo模拟结果显示其具有... 相似文献