全文获取类型
收费全文 | 103789篇 |
免费 | 9969篇 |
国内免费 | 8901篇 |
专业分类
化学 | 30961篇 |
晶体学 | 397篇 |
力学 | 15592篇 |
综合类 | 1815篇 |
数学 | 39980篇 |
物理学 | 33914篇 |
出版年
2023年 | 899篇 |
2022年 | 1487篇 |
2021年 | 1967篇 |
2020年 | 2412篇 |
2019年 | 2446篇 |
2018年 | 2212篇 |
2017年 | 2820篇 |
2016年 | 3220篇 |
2015年 | 2694篇 |
2014年 | 4174篇 |
2013年 | 7166篇 |
2012年 | 4580篇 |
2011年 | 5372篇 |
2010年 | 4602篇 |
2009年 | 6091篇 |
2008年 | 6401篇 |
2007年 | 6639篇 |
2006年 | 6048篇 |
2005年 | 5271篇 |
2004年 | 4824篇 |
2003年 | 4693篇 |
2002年 | 4201篇 |
2001年 | 3621篇 |
2000年 | 3333篇 |
1999年 | 3004篇 |
1998年 | 2760篇 |
1997年 | 2261篇 |
1996年 | 2006篇 |
1995年 | 1975篇 |
1994年 | 1757篇 |
1993年 | 1463篇 |
1992年 | 1437篇 |
1991年 | 1119篇 |
1990年 | 948篇 |
1989年 | 768篇 |
1988年 | 693篇 |
1987年 | 587篇 |
1986年 | 484篇 |
1985年 | 568篇 |
1984年 | 570篇 |
1983年 | 298篇 |
1982年 | 441篇 |
1981年 | 462篇 |
1980年 | 333篇 |
1979年 | 335篇 |
1978年 | 248篇 |
1977年 | 231篇 |
1976年 | 166篇 |
1974年 | 115篇 |
1973年 | 120篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
以轻烧白云石粉、氯化铵和二氧化碳为原料,在未使用晶型控制剂的情况下,通过蒸氨-沉钙过程制备出了似立方体状碳酸钙。研究了反应温度、溶液中钙离子浓度、通气速率、搅拌速度以及陈化时间对碳酸钙中方解石相含量以及晶体形貌的影响,并探索了沉钙反应的晶型控制机理。结果表明,在反应温度40 ℃、钙离子浓度0.05 mol/L、通碳速率100 mL/min、搅拌速度400 r/min和陈化时间2 h的条件下,制备出形貌规整、粒径分布均匀的似立方体状碳酸钙,平均粒径为5~10 μm。该研究为提升白云石的使用价值、生产高附加价值的碳酸钙产品,以及提高白云石资源的利用率提供理论基础。 相似文献
32.
《Operations Research Letters》2022,50(2):137-144
In mine planning problems, cutoff grade optimization defines a threshold at every time period such that material above this value is processed, and the rest is considered waste. In orebodies with multiple minerals, which occur in practice, the natural extension is to consider a cutoff surface. We show that in two dimensions the optimal solution is a line, and in n dimensions it is a hyperplane. 相似文献
33.
Two-stage ignition exists in the low-temperature combustion process of n-heptane and the first-stage ignition also shows a negative temperature coefficient(NTC) phenomenon. To study key reactions and understand chemical principles affecting the first-stage ignition of n-heptane, a lumped skeletal mechanism with 62 species is obtained based on the detailed NUIGMech1.0 mechanism using the directed relation graph method assisted by sensitivity analysis and isomer lumping. The lumped mechanism shows good performance on ignition delay time under wide conditions. The study revealed that the temperature after the first-stage ignition is higher and a larger amount of fuel is consumed at lower initial temperatures. The temperature at the first-stage ignition is relatively insensitive to the initial temperature. Further sensitivity analysis and reaction path analysis carried out based on the lumped mechanism show that the decomposition of RO2 to produce alkene and HO2is the most important reaction to inhibit the first-stage ignitions. The chain branching explosion closely related to the first-stage ignition will be terminated when the rate constant for the RO2 decomposition is larger than that of the isomerization of RO2 to produce QOOH. The NTC behavior as well as other characteristics of the first-stage ignition can be rationalized from the competition between these two reactions. 相似文献
34.
35.
In this article, we construct and analyze a residual-based a posteriori error estimator for a quadratic finite volume method (FVM) for solving nonlinear elliptic partial differential equations with homogeneous Dirichlet boundary conditions. We shall prove that the a posteriori error estimator yields the global upper and local lower bounds for the norm error of the FVM. So that the a posteriori error estimator is equivalent to the true error in a certain sense. Numerical experiments are performed to illustrate the theoretical results. 相似文献
36.
37.
《Indagationes Mathematicae》2022,33(3):682-717
Motivated by a search for Lie group structures on groups of Poisson diffeomorphisms, we investigate linearizability of Poisson structures of Poisson groupoids around the unit section. After extending the Lagrangian neighbourhood theorem to the setting of cosymplectic Lie algebroids, we establish that dual integrations of triangular bialgebroids are always linearizable. Additionally, we show that the (non-dual) integration of a triangular Lie bialgebroid is linearizable whenever the -matrix is of so-called cosymplectic type. The proof relies on the integration of a triangular Lie bialgebroid to a symplectic LA-groupoid, and in the process we define interesting new examples of double Lie algebroids and LA-groupoids. We also show that the product Poisson groupoid can only be linearizable when the Poisson structure on the unit space is regular. 相似文献
38.
39.
为了改善GaN HEMT的自热效应,集成高热导率的金刚石衬底有助于增强器件有源区的热量耗散。然而,化学气相淀积(CVD)生长的多晶金刚石(PCD)具有柱状晶粒结构,导致了各向异性的材料热导率,且其热导率值与生长厚度有关。为此,通过建模金刚石生长过程中晶粒尺寸的演变过程,计算了金刚石沿面内和截面方向的热导率。基于该PCD热导率模型,利用计入材料非线性热导率的GaN器件热阻解析模型,计算得到了GaN HEMT沟道温度的波动范围,并分析了其与器件结构(栅长、栅宽、栅间距、衬底厚度)和功耗的依赖关系。最后,通过与有限元(FEM)仿真结果对比,分区域提取了GaN HEMT器件中PCD衬底的有效热导率,分别为260~310 W/(m·K)和1 250~1 450 W/(m·K)。本文的计算为预测金刚石衬底上GaN HEMT器件的沟道温度提供了快速、有效的方法。 相似文献
40.
《Journal of Pure and Applied Algebra》2022,226(8):106948
Let be a Henselian discrete valued field with residue field of characteristic , and be the Brauer p-dimension of K. This paper shows that if , for some . It proves that if and only if . 相似文献