全文获取类型
收费全文 | 67827篇 |
免费 | 5231篇 |
国内免费 | 2868篇 |
专业分类
化学 | 6942篇 |
晶体学 | 193篇 |
力学 | 9410篇 |
综合类 | 328篇 |
数学 | 7544篇 |
物理学 | 10568篇 |
综合类 | 40941篇 |
出版年
2024年 | 162篇 |
2023年 | 610篇 |
2022年 | 1043篇 |
2021年 | 1190篇 |
2020年 | 1337篇 |
2019年 | 1147篇 |
2018年 | 1181篇 |
2017年 | 1621篇 |
2016年 | 1778篇 |
2015年 | 2150篇 |
2014年 | 2975篇 |
2013年 | 3917篇 |
2012年 | 3621篇 |
2011年 | 3823篇 |
2010年 | 2969篇 |
2009年 | 3507篇 |
2008年 | 3542篇 |
2007年 | 4329篇 |
2006年 | 3882篇 |
2005年 | 3499篇 |
2004年 | 3334篇 |
2003年 | 2987篇 |
2002年 | 2609篇 |
2001年 | 2212篇 |
2000年 | 2083篇 |
1999年 | 1862篇 |
1998年 | 1611篇 |
1997年 | 1534篇 |
1996年 | 1365篇 |
1995年 | 1258篇 |
1994年 | 1050篇 |
1993年 | 952篇 |
1992年 | 818篇 |
1991年 | 704篇 |
1990年 | 637篇 |
1989年 | 574篇 |
1988年 | 525篇 |
1987年 | 377篇 |
1986年 | 260篇 |
1985年 | 187篇 |
1984年 | 124篇 |
1983年 | 66篇 |
1982年 | 124篇 |
1981年 | 62篇 |
1980年 | 63篇 |
1979年 | 67篇 |
1978年 | 36篇 |
1977年 | 38篇 |
1976年 | 45篇 |
1957年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
《Mendeleev Communications》2022,32(5):597-600
Calorimetric monitoring of the autoclave reaction N2O4 + C2H4 at –85 to +10 °C under argon pressure 10–30 bar revealed that the exothermic chemical reaction started at temperatures above –52 °C at 10 bar, whereas an intensive exothermic reaction started at –85 °C and pressure of 30 bar. IR study showed that oligo/polynitroethylene was formed at 30 bar, while carbonyl and hydroxy compound as well as nitrate R–ONO2 formation occurred upon processing at 10 bar. 相似文献
2.
当煤层上方为坚硬顶板时,在工作面回采期间容易出现大面积悬顶,导致巷旁充填体出现大变形破坏,极大威胁采煤安全.为解决这一问题,以某矿1205工作面沿空留巷为工程背景,分析了爆炸围岩裂隙范围和坚硬顶板破断特征,建立了切顶卸压围岩结构力学模型,研究表明,相邻炮孔爆炸后,炮孔间裂隙互相贯穿,降低了顶板强度,同时当切顶角为15°时,巷道稳定性较好.工程实践结果表明:采用爆破切顶技术后,顶板下沉量减小了62.3%,较好地保证了围岩稳定性. 相似文献
3.
4.
An efficient edge based data structure has been developed in order to implement an unstructured vertex based finite volume algorithm for the Reynolds-averaged Navier–Stokes equations on hybrid meshes. In the present approach, the data structure is tailored to meet the requirements of the vertex based algorithm by considering data access patterns and cache efficiency. The required data are packed and allocated in a way that they are close to each other in the physical memory. Therefore, the proposed data structure increases cache performance and improves computation time. As a result, the explicit flow solver indicates a significant speed up compared to other open-source solvers in terms of CPU time. A fully implicit version has also been implemented based on the PETSc library in order to improve the robustness of the algorithm. The resulting algebraic equations due to the compressible Navier–Stokes and the one equation Spalart–Allmaras turbulence equations are solved in a monolithic manner using the restricted additive Schwarz preconditioner combined with the FGMRES Krylov subspace algorithm. In order to further improve the computational accuracy, the multiscale metric based anisotropic mesh refinement library PyAMG is used for mesh adaptation. The numerical algorithm is validated for the classical benchmark problems such as the transonic turbulent flow around a supercritical RAE2822 airfoil and DLR-F6 wing-body-nacelle-pylon configuration. The efficiency of the data structure is demonstrated by achieving up to an order of magnitude speed up in CPU times. 相似文献
5.
Prediction of drag reduction effect caused by pulsating pipe flows is examined using machine learning. First, a large set of flow field data is obtained experimentally by measuring turbulent pipe flows with various pulsation patterns. Consequently, more than 7000 waveforms are applied, obtaining a maximum drag reduction rate and maximum energy saving rate of 38.6% and 31.4%, respectively. The results indicate that the pulsating flow effect can be characterized by the pulsation period and pressure gradient during acceleration and deceleration. Subsequently, two machine learning models are tested to predict the drag reduction rate. The results confirm that the machine learning model developed for predicting the time variation of the flow velocity and differential pressure with respect to the pump voltage can accurately predict the nonlinearity of pressure gradients. Therefore, using this model, the drag reduction effect can be estimated with high accuracy. 相似文献
6.
7.
8.
针对车辆在钩形弯交叉口与相邻常规交叉口间的整体运行特性,建立了各条车道的车辆平均延误计算模型。以各交叉口的周期时长和相位绿灯时间为约束,钩形弯交叉口与相邻常规交叉口间的车辆平均延误最小为目标,建立了信号协同配时方案。以位于苏州市高新区内有轨电车沿线的两个相邻交叉口为例,采用实际数据验证所建立的方法,并设置钩形弯渠化方案。将设置的钩形弯交叉口与其相邻常规交叉口的信号协同作为改进方法,在VISSIM软件中对比分析改进方案与现状方案的车辆延误指标。实验结果表明,钩形弯交叉口与相邻常规交叉口的信号协同优化方法可有效提高交叉口的通行能力,尤其是干道直行机动车的通行能力,可降低车辆的平均延误。 相似文献
9.
建立了整车8-DOF系统动力学模型,考虑了主动悬架控制,并增设了主动座椅控制,设计了车辆主动悬架系统的LQG控制器。基于Matlab仿真平台建立了整车8-DOF系统动力学仿真模型,对所得最优控制策略下的动态响应进行了仿真验证。仿真结果表明:为了改善人椅系统质心及车身质心的跳振性能需要在一定程度上弱化各轮轮胎动位移性能。从控制效能上来看,该最优控制器能够满足各行驶状态下对悬架性能的要求,改善了车辆的行驶平顺性。 相似文献
10.
Cavitation erosion is caused in solids exposed to strong pressure waves developing in an adjacent fluid field. The knowledge of the transient distribution of stresses in the solid is important to understand the cause of damaging by comparisons with breaking points of the material. The modeling of this problem requires the coupling of the models for the fluid and the solid. For this purpose, we use a strategy based on the solution of coupled Riemann problems that has been originally developed for the coupling of 2 fluids. This concept is exemplified for the coupling of a linear elastic structure with an ideal gas. The coupling procedure relies on the solution of a nonlinear equation. Existence and uniqueness of the solution is proven. The coupling conditions are validated by means of quasi‐1D problems for which an explicit solution can be determined. For a more realistic scenario, a 2D application is considered where in a compressible single fluid, a hot gas bubble at low pressure collapses in a cold gas at high pressure near an adjacent structure. 相似文献