全文获取类型
收费全文 | 161912篇 |
免费 | 24702篇 |
国内免费 | 17535篇 |
专业分类
化学 | 83907篇 |
晶体学 | 1518篇 |
力学 | 8012篇 |
综合类 | 882篇 |
数学 | 14437篇 |
物理学 | 48258篇 |
无线电 | 47135篇 |
出版年
2024年 | 615篇 |
2023年 | 3643篇 |
2022年 | 4711篇 |
2021年 | 5956篇 |
2020年 | 5967篇 |
2019年 | 5659篇 |
2018年 | 4871篇 |
2017年 | 4838篇 |
2016年 | 6920篇 |
2015年 | 7387篇 |
2014年 | 8986篇 |
2013年 | 11553篇 |
2012年 | 13543篇 |
2011年 | 13924篇 |
2010年 | 10304篇 |
2009年 | 10158篇 |
2008年 | 10720篇 |
2007年 | 9842篇 |
2006年 | 9261篇 |
2005年 | 7897篇 |
2004年 | 5636篇 |
2003年 | 4727篇 |
2002年 | 4260篇 |
2001年 | 3541篇 |
2000年 | 3219篇 |
1999年 | 3467篇 |
1998年 | 2956篇 |
1997年 | 2537篇 |
1996年 | 2765篇 |
1995年 | 2233篇 |
1994年 | 2089篇 |
1993年 | 1684篇 |
1992年 | 1548篇 |
1991年 | 1361篇 |
1990年 | 1065篇 |
1989年 | 745篇 |
1988年 | 628篇 |
1987年 | 472篇 |
1986年 | 467篇 |
1985年 | 401篇 |
1984年 | 295篇 |
1983年 | 224篇 |
1982年 | 183篇 |
1981年 | 124篇 |
1980年 | 106篇 |
1979年 | 71篇 |
1978年 | 63篇 |
1977年 | 51篇 |
1976年 | 49篇 |
1974年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
741.
§ 1 IntroductionInthispaperwediscusstheglobalclassicalsolutionofamultidimensionalquasistationaryproblem .Theproblemcomesfromthediscussionofagrowthmodelofselfmaintainingprotocell(see [1— 3])inmultidimensionalcase .Theprotocellcanbevisualizedashavingaporousst… 相似文献
742.
Various problems associated with optimal path planning for mobile observers such as mobile robots equipped with cameras to obtain maximum visual coverage of a surface in the three-dimensional Euclidean space are considered. The existence of solutions to these problems is discussed first. Then, optimality conditions are derived by considering local path perturbations. Numerical algorithms for solving the corresponding approximate problems are proposed. Detailed solutions to the optimal path planning problems for a few examples are given. 相似文献
743.
在超高真空分子束外延(MBE)生长技术中,反射式高能电子衍射仪(RHEED)能实时显示半导体和金属外延生长过程,给出薄膜表面结构和平整度的信息,成为MBE必备的原位表面分 析仪.为了研究氧化物薄膜如高温超导(YBa2Cu3O7) 、铁电薄膜(Sr1-xBax TiO3)及它们的同质和异质外延结构的生长机理,获得高质量的符合各种应用 需要的氧化 物多层薄膜结构,在常规的制备氧化
关键词:
高温超导薄膜
RHEED 相似文献
744.
A tight-binding (TB) hamiltonian for calcium is built with a high precision parametrization technique based on density functional calculations of the energy bands and the total energy at various lattice volumes. The new set of TB parameters is appropriate to study phenomena under pressures as high as 20 GPa. Specifically, both the metal to nonmetal transition at 4 GPa and the structural transition fcc to bcc at 19 GPa are well reproduced. These transitions and several static properties are in excellent agreement with experiments. Phonon frequencies, plasmon energy, melting temperature and the coefficient of thermal expansion were calculated with a molecular dynamics scheme of this TB hamiltonian. 相似文献
745.
Hong‐Qing Wang Zhao‐Jie Liu Li‐Min Yang Ming‐Wu Ding 《Journal of heterocyclic chemistry》2004,41(3):393-397
A series of new 1H‐pyrazolo[3,4‐d]pyrimidin‐4(5H)‐one Derivatives 5 has been designed and regio‐selectively synthesized via a tandem aza‐Wittig reaction. The structures of all compounds prepared have been confirmed by 1H NMR, IR, EI‐MS spectroscopy and elemental analyses. The results of preliminary bioassay indicate that most compounds 5 possess an inhibition effect against Botrytis cinereapers and Pyricularia oryzae at the concentration of 50 mg/L. 相似文献
746.
Piaoping Yang Jianfeng Yu Zhenlü Wang Qingsheng Liu Tonghao Wu 《Reaction Kinetics and Catalysis Letters》2004,83(2):275-282
Mg-Al, Zn-Al and Ni-Al hydrotalcite-like compounds with high crystallinity were synthesized by using the decomposing property
of urea at high temperature. The crystallinity and the regularly of the samples were much preferable to those synthesized
by other methods.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
747.
The postulate of a linear tensor relation in the measured equation of invariance (MEI) is proposed for three-dimensional (3D) problems. As a result, all three components of the field vector to be solved are coupled in the MEI. Combined with the finite difference method, the present method is applied to the analysis of scattering by 3D conducting objects and results in a significant improvement in the accuracy of numerical results as compared to those obtained with an uncoupled linear relation. 相似文献
748.
交流伺服系统是一种复杂的非线性时变系统。文中介绍了应用MATLAB/SIMULINK可视化动态仿真平台进行软件伺服的系统仿真方法。该方法采用易扩展的模块化设计,可在线修改和观察所有参数以及参数变化对系统控制效果的影响,从而能方便地验证各种控制策略,并据此选出高效的快速设计方案,缩短了研制周期,对系统设计和调试起到了很大的作用。 相似文献
749.
Kim S. Wang Y. E. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(12):2521-2528
Transient characteristics of switched resonators have been studied. The resonators exhibit controllable impedance and energy storage behaviors during an alternating charging and discharging process. It is discovered that these unique characteristics of switched resonators can lead to novel RF envelope signal processing functions. The paper discusses the basic theory of switched resonators and the designing methodology of relevant RF signal processing applications. Two applications are presented as examples. The first example is a variable, reflective attenuator which uses the controllable impedance of the switched resonators to achieve different attenuation/reflection levels. In the second example, a simple, compact and efficient RF pulse compression technique based on the switched resonator concept is introduced. It utilizes the charging/discharging characteristics of the circuit resonator by varying the resonator's Q-factor. The approach compresses the duration of the modulated pulse without changing the phase characteristics of the RF carrier. The pulse compression technique can also be implemented with other type of resonators including distributed resonators, as demonstrated through the experiments 相似文献
750.