全文获取类型
收费全文 | 122046篇 |
免费 | 12341篇 |
国内免费 | 7240篇 |
学科分类
工业技术 | 141627篇 |
出版年
2024年 | 613篇 |
2023年 | 1978篇 |
2022年 | 4033篇 |
2021年 | 5570篇 |
2020年 | 4027篇 |
2019年 | 3304篇 |
2018年 | 3604篇 |
2017年 | 4066篇 |
2016年 | 3827篇 |
2015年 | 5481篇 |
2014年 | 6566篇 |
2013年 | 7833篇 |
2012年 | 8992篇 |
2011年 | 9567篇 |
2010年 | 8640篇 |
2009年 | 8100篇 |
2008年 | 8002篇 |
2007年 | 7238篇 |
2006年 | 6978篇 |
2005年 | 5963篇 |
2004年 | 4152篇 |
2003年 | 3361篇 |
2002年 | 3542篇 |
2001年 | 2983篇 |
2000年 | 2519篇 |
1999年 | 2122篇 |
1998年 | 1524篇 |
1997年 | 1304篇 |
1996年 | 1201篇 |
1995年 | 1049篇 |
1994年 | 825篇 |
1993年 | 640篇 |
1992年 | 495篇 |
1991年 | 362篇 |
1990年 | 278篇 |
1989年 | 212篇 |
1988年 | 184篇 |
1987年 | 108篇 |
1986年 | 93篇 |
1985年 | 55篇 |
1984年 | 42篇 |
1983年 | 25篇 |
1982年 | 33篇 |
1981年 | 35篇 |
1980年 | 32篇 |
1979年 | 12篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1959年 | 13篇 |
1951年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
设计了一种适合在低电源电压下工作的前馈型输入级放大结构,在全摆幅的动态工作范围内,输入级跨导保持不变,采用负载电流补偿以保证增益近似恒定,输出采用前馈型AB类输结构,实现全摆幅输出。 相似文献
72.
A formulation is presented for a two-dimensional time-domain finite-element method (FEM-TD) that incorporates periodic boundaries. The specifics of the method are shown for scattering problems, but it should be straightforward to extend it to radiation problems. The method solves for a transformed field variable (instead of solving directly for the electric field) in order to easily enable periodic boundary conditions in the time domain. The accuracy and stability of the method is demonstrated by a series of examples where the new formulation is compared with reference solutions. Very accurate results are obtained when the excitation (frequency range) and the geometry are such that no higher order Floquet modes are present. The accuracy is degraded in the presence of higher order modes due to the rather simple absorbing boundary condition that is used with the present formulation. The method is found to be stable even for angles of incidence close to grazing. 相似文献
73.
In this paper we design an experiment which can be depicted as a simple scenario, a very limited 'world'. In this world, there are an actor that can pursue a project and an observer that is keeping its eyes on the actor. We try to show in the experiment that the observer can to some degree understand the actor based on its knowledge and some metaphors, i.e. understand what the actor is doing and why. As the conclusion of this experiment, we try to show some features of 'understanding'. These are (1) that 'understanding' has to be based on some preliminary knowledge; (2) that 'understanding' is a process of incremental learning; (3) that, as for symbolic systems, some metaphors are necessary for mapping real entities into concepts in mind. 相似文献
74.
75.
76.
Study on Oxygen Isotope Fractionation in RE Oxide Minerals 总被引:2,自引:0,他引:2
StudyonOxygenIsotopeFractionationinREOxideMineralsFangTao(方涛),QiuYuzhuo(裘愉卓)(InstituteofGeochemistry,AcademiaSinica,Guiyang5... 相似文献
77.
OBJECTIVE: To identify the influence of the increased level of serum cholyglycine (CG) on lipid metabolism and hemorrheology in patients with intrahepatic cholestasis during pregnancy (ICP). METHODS: The concentrations of serum CG, total cholesterol (CH), triglycerides (TG), high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol (LDL-C), high shear and low shear of blood specific viscosity (HS and LS), plasma specific viscosity (PV) and hematocrit (HCT) were measured in 68 cases of primiparas with single pregnancy and 30 healthy nonpregnant women respectively. The group of ICP was composed of 35 cases with CG > 6 mumol/L, the group of normal pregnancy, 33 cases with CG < 6 mumol/L. RESULTS: The means of CG and the lipidic parameters in the two pregnant groups were significantly higher than those in the non-pregnancy group (P < 0.02-0.001) except the means of HDL-C between the groups of ICP and non-pregnancy. The levels of LDL-C, LDL-C/HDL-C, LS, PV and HCT in the ICP were significantly higher than those in the normal pregnancy group (P < 0.02-0.001). CONCLUSIONS: There are significant changes of lipid metabolism and hemorrheology in patients with ICP. However, these changes could be corrected after pregnancy termination, when the level of serum CG returned to normal. The results suggest that the pathophysiologic changes of ICP are associated with increased level of serum CG. 相似文献
78.
新型超微晶软磁合金FePCCuMoSi微观结构缺陷的正电子湮没研究 总被引:1,自引:0,他引:1
用正电子湮没方法研究了新型超微晶软磁合金的微观结构缺陷,样品选用的是在不同温度下退火的Fe81P12C3Cu1Mo0.5Si2.5合金,结果表明,合金的微观结构构陷大小,密度随退火温度有规律地变化,这种现象可能与非晶的晶化过程有关。 相似文献
79.
Electromigration reliability of interconnect under bidirectional current stress has been studied in a wide frequency range (mHz to 200 MHz). Experimental results show that the AC lifetime rises with the stress current frequency. The current density exponent and the activation energy of AC lifetime are found to be twice that of DC lifetime. Pure AC current stress failure at extremely high current density is believed to result from thermal migration of metal at hot/cold transition points 相似文献
80.
Fractal geometry is finding its application in the nondestructive evaluation (NDE) of corrosion by characterizing the corrosion surface with fractal dimensions. It was found that corrosion surfaces can be well described by multifractals. Two algorithms were used to obtain the fractal dimensions of a corroded aluminum alloy surface, one being box-counting and another being coherent light scattering. Fractal dimensions were related with the damage caused by corrosion and fatigue. 相似文献