全文获取类型
收费全文 | 691828篇 |
免费 | 8830篇 |
国内免费 | 1810篇 |
专业分类
电工技术 | 12522篇 |
综合类 | 755篇 |
化学工业 | 106180篇 |
金属工艺 | 26219篇 |
机械仪表 | 20266篇 |
建筑科学 | 16641篇 |
矿业工程 | 3369篇 |
能源动力 | 17952篇 |
轻工业 | 63252篇 |
水利工程 | 6976篇 |
石油天然气 | 12629篇 |
武器工业 | 68篇 |
无线电 | 76266篇 |
一般工业技术 | 131439篇 |
冶金工业 | 136517篇 |
原子能技术 | 14353篇 |
自动化技术 | 57064篇 |
出版年
2021年 | 6252篇 |
2020年 | 4482篇 |
2019年 | 5833篇 |
2018年 | 9903篇 |
2017年 | 9686篇 |
2016年 | 10274篇 |
2015年 | 6990篇 |
2014年 | 11646篇 |
2013年 | 31189篇 |
2012年 | 18494篇 |
2011年 | 25373篇 |
2010年 | 20030篇 |
2009年 | 22397篇 |
2008年 | 22955篇 |
2007年 | 22808篇 |
2006年 | 20037篇 |
2005年 | 18195篇 |
2004年 | 17480篇 |
2003年 | 17143篇 |
2002年 | 16442篇 |
2001年 | 16417篇 |
2000年 | 15400篇 |
1999年 | 16127篇 |
1998年 | 41344篇 |
1997年 | 28940篇 |
1996年 | 22165篇 |
1995年 | 16741篇 |
1994年 | 14622篇 |
1993年 | 14284篇 |
1992年 | 10454篇 |
1991年 | 9732篇 |
1990年 | 9822篇 |
1989年 | 9488篇 |
1988年 | 8895篇 |
1987年 | 7914篇 |
1986年 | 7827篇 |
1985年 | 8816篇 |
1984年 | 8276篇 |
1983年 | 7370篇 |
1982年 | 6961篇 |
1981年 | 7078篇 |
1980年 | 6768篇 |
1979年 | 6554篇 |
1978年 | 6428篇 |
1977年 | 7576篇 |
1976年 | 9922篇 |
1975年 | 5611篇 |
1974年 | 5270篇 |
1973年 | 5440篇 |
1972年 | 4493篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
152.
High division of the brachial artery was observed in two cadavers, during routine dissection of upper extremities. In the first case, the brachial artery of the right upper extremity was divided into its two terminal branches immediately after passing between the lateral and medial roots of the median nerve and just below the origin of the profunda brachii artery. The lateral branch was the radial artery, located in the space normally occupied by the brachial artery and the medial one was the ulnar artery. In the second case, the brachial artery was divided into its two terminal branches just below the origin of the profunda brachii artery. Accurate knowledge of the relationships and course of these major arterial conduits and particularly of their variational patterns, is of considerable practical importance in the conduct of reperative surgery in the arm, forearm and hand. 相似文献
153.
The objective of this study is to determine the economic and operational impact on energy cost of incorporating large photovoltaic (PV) and wind energy conversion systems (WECS) into the electric utility generation mix. In most cases, PV and WECS power outputs are subtracted from the utility load with the expectation that conventional generation would meet the residual load. This approach is valid for small penetration levels and/or for PV and WECS facilities connected near load centers, However, several constraints such as thermal generation characteristics, fuel supply and delivery, spinning reserve requirements, and hydro availability are not adequately represented in this process. To determine the optimal value of large-scale PV and WECS applications, a new methodology that would take into account the aforementioned constraints as well as a more global penetration is developed. Results indicate that while high hydro availability increases PV penetration levels, high ramping rates can also significantly increase penetration levels 相似文献
154.
Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism 相似文献
155.
Auld B.A. Chimenti D.E. Shull P.J. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1996,43(2):319-325
The transverse resonance approach to guided wave analysis is applied to shear horizontal (SH) wave propagation in periodically layered composites. It is found for SH waves that at high values of the guided wavevector β, the wave energy is trapped in the slower of the two media and propagates accordingly at the slower wavespeed. At low values of β, however, the modes demonstrate a clustering behavior, indicative of the underlying Floquet wave structure. The number of modes in a cluster is observed to correlate with the number of unit cells in the layered plate. New physical insights into the behavior of these systems are obtained by analyzing the partial waves of the guided SH modes in terms of Floquet waves. We show that the fast and slow shear waves in the periodically layered composite play an analogous role to the longitudinal and shear partial waves comprising Lamb waves in a homogeneous plate 相似文献
156.
A low threshold current density of ~100 A/cm2 has been obtained at 1.55 μm using a graded-index separate-confinement-heterostructure strained InGaAsP single-quantum-well laser. The design of the laser structure is based on results calculated from the viewpoint of effective carrier injection into the well 相似文献
157.
158.
159.
160.
Data from the past few years have shown that as caffeine metabolizes solely in the liver, caffeine elimination can serve as a liver function test. We have collected data by monitoring 40 persons with liver diseases (11 chronic alcoholic hepatitis, 24 liver cirrhosis, 5 non-cirrhotic liver disease). Eight subjects served as controls. The patients with liver cirrhosis were classified according to the Child--Pugh scoring system. To determine caffeine elimination blood samples were collected before and at 3, 6, 9 and 12 hours after oral administration of 0.2 g caffeine. Fasting serum caffeine concentration and concentration 12 hours after administration, serum clearance, half life, peak concentration and volume of distribution have been compared. The respective values measured in patients with non-cirrhotic liver diseases did not differ significantly from the controls. The disappearance of caffeine was significantly decreased in cirrhotics. Our results demonstrated a good correlation between impairment of caffeine elimination and assessment of severity of liver disease by the Child--Pugh classification. Measuring serum levels in samples taken 12 hours after caffeine administration is a simple and useful method in the diagnosis of liver diseases at cirrhotic stage. 相似文献