全文获取类型
收费全文 | 4290篇 |
免费 | 311篇 |
国内免费 | 263篇 |
专业分类
化学 | 2727篇 |
晶体学 | 39篇 |
力学 | 115篇 |
综合类 | 28篇 |
数学 | 770篇 |
物理学 | 524篇 |
综合类 | 661篇 |
出版年
2023年 | 38篇 |
2022年 | 57篇 |
2021年 | 55篇 |
2020年 | 123篇 |
2019年 | 96篇 |
2018年 | 103篇 |
2017年 | 88篇 |
2016年 | 148篇 |
2015年 | 122篇 |
2014年 | 138篇 |
2013年 | 230篇 |
2012年 | 155篇 |
2011年 | 190篇 |
2010年 | 183篇 |
2009年 | 184篇 |
2008年 | 168篇 |
2007年 | 204篇 |
2006年 | 181篇 |
2005年 | 223篇 |
2004年 | 207篇 |
2003年 | 174篇 |
2002年 | 167篇 |
2001年 | 131篇 |
2000年 | 166篇 |
1999年 | 121篇 |
1998年 | 110篇 |
1997年 | 112篇 |
1996年 | 90篇 |
1995年 | 103篇 |
1994年 | 84篇 |
1993年 | 64篇 |
1992年 | 62篇 |
1991年 | 62篇 |
1990年 | 63篇 |
1989年 | 52篇 |
1988年 | 50篇 |
1987年 | 53篇 |
1986年 | 43篇 |
1985年 | 49篇 |
1984年 | 37篇 |
1983年 | 29篇 |
1982年 | 33篇 |
1981年 | 32篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有4864条查询结果,搜索用时 15 毫秒
961.
利用混沌动力学的分析手段,对一个包含了地形、热力强迫以及耗散作用的准静止超长波动力系统的行为进行了初步探讨.结果表明,超长波平衡态的建立与加热场的结构有关.方程的定性分析及数值计算结果均表明,当超长波表现为周期行为时,其周期约为30~40d,与低频振荡的频率相当.另外,还给出了一个与地形、耗散以及加热等因子有关的判据Cr,当Cr较小时系统出现周期态,而当Cr较大时系统出现混沌特征. 相似文献
962.
张克伟 《内蒙古大学学报(自然科学版)》1995,(6)
考虑于1968年作为粘性不可压缩流的一个数学模型提出的修正的Navier-Stokes方程的定常解的存在性,唯一性和吸引性.定常的修正的Navier-Stokes方程是满足一定的单调性条件的拟线性椭圆方程组.利用逼近建立了一个一般的存在性定理,进而看到,如果或者椭圆性常数足够大,或者具适当大的单调性参数,或者外力相当小,则有唯一的定常解,最后,我们在类似(仅仅是类似!)于上述的条件下证明了所有的定常解的集合是极小的紧的不变的可吸引相空间中任何有界集的吸引子. 相似文献
963.
孟庆余 《南京邮电大学学报(自然科学版)》2012,32(4):110-112
构造了一个在闭区间上连续,开区间内可导的严格单调可微函数,其驻点集与实数集等势。该函数说明在闭区间上连续,开区间内可导的严格单调可微函数,其驻点的个数可以相当"多"。 相似文献
964.
本文通过对在专门研制的环盘式双列叶栅实验装置上所进行的固定导叶与活动导叶实验的分析,认为水轮机的固定导叶与活动导叶圆周方向的相互位置对导叶区域水流的特性和水力损失影响较大,指出了对称型和正曲率型活动导叶与固定导叶的最佳相对安放位置和流场的不同特性;建立了水轮机设计时确定固定导叶与活动导叶相对安放位置的经验公式;并分析了高水头低比转速水轮机导叶汽蚀破坏的主要水力原因。 相似文献
965.
In this paper, based on the stabilization technique, the Oseen iterative method and the two-level finite element algorithm are combined to numerically solve the stationary incompressible magnetohydrodynamic (MHD) equations. For the low regularity of the magnetic field, when dealing with the magnetic field sub-problem, the Lagrange multiplier technique is used. The stabilized method is applied to approximate the flow field sub-problem to circumvent the inf-sup condition restrictions. One- and two-level stabilized finite element algorithms are presented, and their stability and convergence analysis is given. The two-level method uses the Oseen iteration to solve the nonlinear MHD equations on a coarse grid of size H, and then employs the linearized correction on a fine grid with grid size h. The error analysis shows that when the grid sizes satisfy , the two-level stabilization method has the same convergence order as the one-level one. However, the former saves more computational cost than the latter one. Finally, through some numerical experiments, it has been verified that our proposed method is effective. The two-level stabilized method takes less than half the time of the one-level one when using the second class Nédélec element to approximate magnetic field, and even takes almost a third of the computing time of the one-level one when adopting the first class Nédélec element. 相似文献
966.
Testing predictability is known to be an important issue for the balanced predictive regression model. Some unified testing statistics of desirable properties have been proposed, though their validity depends on a predefined assumption regarding whether or not an intercept term nevertheless exists. In fact, most financial data have endogenous or heteroscedasticity structure, and the existing intercept term test does not perform well in these cases. In this paper, we consider the testing for the intercept of the balanced predictive regression model. An empirical likelihood based testing statistic is developed, and its limit distribution is also derived under some mild conditions. We also provide some simulations and a real application to illustrate its merits in terms of both size and power properties. 相似文献
967.
Mehdi Shahedi Asl Abbas Sabahi Namini Seyed Ali Delbari Quyet Van Le Mohammadreza Shokouhimehr Mohsen Mohammadi 《自然科学进展(英文版)》2021,31(1):47-54
Sintering behavior of ZrB2 ceramic with nano-sized SiC dopant was studied. ZrB2-25 vol% nano-sized SiC was selected as the starting mixture to fabricate the composite. The manufacturing process was accomplished at 1800℃ for 5 min under 25 MPa via spark plasma sintering(SPS). The as-sintered sample reached a relative density of 99%. Besides the initial phases, namely ZrB2 and SiC, the high-resolution X-ray diffraction(HRXRD) was used to study the formation of an i... 相似文献
968.
Ina Varfaj Andrea Carotti Luciano Mangiapelo Lina Cossignani Agnese Taticchi Antonio Macchiarulo Federica Ianni Roccaldo Sardella 《Molecules (Basel, Switzerland)》2022,27(22)
Two LC methods were developed for the achiral and chiral reversed-phase (RP) analysis of an amino acid (AA) pool in a food supplement, in compliance with the main paradigms of Green Chromatography. A direct achiral ion-pairing RP-HPLC method was optimized under gradient conditions with a water-ethanol (EtOH) eluent containing heptafluorobutyric acid (0.1%, v/v), to quantify the eight essential AAs (Ile, Leu, Lys, Met, Phe, Thr, Trp, and Val) contained in the food supplement. Thus, the usually employed acetonitrile was profitably substituted with the less toxic and more benign EtOH. The method was validated for Leu and Phe. The chiral LC method performed with a teicoplanin chiral stationary phase was developed with a water-EtOH (60:40, v/v) eluent with 0.1%, v/v acetic acid. The enantioselective analysis was carried out without any prior derivatization step. Both developed methods performed highly for all eight AAs and revealed that: (i) the content of six out of eight AAs was consistent with the manufacturer declaration; (ii) only L-AAs were present. Furthermore, it was demonstrated that a two-dimensional achiral–chiral configuration is possible in practice, making it even more environmentally sustainable. A molecular modelling investigation revealed interesting insights into the enantiorecognition mechanism of Lys. 相似文献
969.
970.
报道了一种基于聚(苯乙烯-二乙烯基苯)微球(PS-DVB)的表面共聚氢氧根选择性阴离子固定相。它以烯丙基缩水甘油醚(AGE)为功能单体、通过自由基引发直接与PS-DVB微球表面残留的悬挂双键共聚,再通过醇胺开环得到。考察了两种醇胺试剂对分离的影响;扫描电镜、红外光谱、元素分析表征结果表明:表面共聚反应成功在微球表面引入季胺基团,且对微球理化性质无显著影响;所得固定相表现氢氧根淋洗液的高选择性,对常规无机阴离子表现出良好的分离性能(分离度>1.5)和运行稳定性(保留时间的相对标准偏差<1.13%),其实用性通过分析茶叶样品中无机阴离子进行了展示。 相似文献