首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7808篇
  免费   367篇
  国内免费   15篇
电工技术   159篇
综合类   21篇
化学工业   1525篇
金属工艺   218篇
机械仪表   402篇
建筑科学   170篇
矿业工程   5篇
能源动力   297篇
轻工业   605篇
水利工程   42篇
石油天然气   21篇
无线电   1457篇
一般工业技术   1357篇
冶金工业   720篇
原子能技术   89篇
自动化技术   1102篇
  2023年   61篇
  2022年   86篇
  2021年   126篇
  2020年   110篇
  2019年   129篇
  2018年   167篇
  2017年   146篇
  2016年   214篇
  2015年   187篇
  2014年   311篇
  2013年   544篇
  2012年   443篇
  2011年   481篇
  2010年   351篇
  2009年   426篇
  2008年   364篇
  2007年   332篇
  2006年   335篇
  2005年   288篇
  2004年   274篇
  2003年   251篇
  2002年   277篇
  2001年   206篇
  2000年   186篇
  1999年   176篇
  1998年   375篇
  1997年   221篇
  1996年   162篇
  1995年   133篇
  1994年   114篇
  1993年   114篇
  1992年   60篇
  1991年   71篇
  1990年   56篇
  1989年   58篇
  1988年   40篇
  1987年   44篇
  1986年   39篇
  1985年   31篇
  1984年   29篇
  1983年   20篇
  1982年   14篇
  1981年   23篇
  1980年   16篇
  1979年   9篇
  1978年   15篇
  1977年   14篇
  1976年   21篇
  1974年   6篇
  1973年   8篇
排序方式: 共有8190条查询结果,搜索用时 13 毫秒
51.
52.
Adipocytic tumors are the most common subtype of soft tissue tumors. In current clinical practice, distinguishing benign lipomas from well-differentiated liposarcomas (WDLPS), as well as dedifferentiated liposarcomas (DDLPS) from their morphologic mimics, remains a significant diagnostic challenge. This is especially so when examining small biopsy samples and without the aid of additional ancillary tests. Recognizing the important role that microRNAs (miRNAs) play in tumorigenesis and their potential utility in tumor classification, we analyzed routine clinical tissue samples of benign and malignant lipomatous tumors, as well as other sarcoma mimics, to identify distinguishing miRNA-based signatures that can aid in the differential diagnosis of these entities. We discovered a 6-miRNA signature that separated lipomas from WDLPS with high confidence (AUC of 0.963), as well as a separate 6-miRNA signature that distinguished DDLPS from their more aggressive histologic mimics (AUC of 0.740). Functional enrichment analysis unveiled possible mechanistic involvement of these predictive miRNAs in adipocytic cancer-related biological processes and pathways such as PI3K/AKT/mTOR and MAPK signaling, further supporting the relevance of these miRNAs as biomarkers for adipocytic tumors. Our results demonstrate that miRNA expression profiling may potentially be used as an adjunctive tool for the diagnosis of benign and malignant adipocytic tumors. Further validation studies are warranted.  相似文献   
53.
The pH variations of Mg, Zn and Al solutions to which had been added the rare earth metals Nd, Pr and Y were observed in 3.5 wt.% NaCl with respect to high energy mechanical ball milling effects. Mg was directly dissolved and exhibited a pH value of 10.5. On the other hand, Zn and Al needed to be saturated for a certain amount of time. The addition of rare earth metals played a role in increasing the pH with low reduction potentials. Additionally, mechanical ball milling provided high energy to Mg + x wt.% Zn + 0.5 wt.% Nd mixture by fracturing fragmentation of metals, which led to an increase in the pH when the mixture was immersed in 3.5 wt.% NaCl. The addition of Zn to Mg + 0.5 wt.% Nd caused a higher pH than when Mg + 0.5 wt.% Nd alone was added.  相似文献   
54.
In this paper, the conduction and switching losses of a voltage-fed three-phase pulsewidth modulation (PWM) rectifier are analyzed for various PWM schemes. On the basis of this result, a novel PWM strategy which minimizes the loss of a three-phase PWM rectifier is developed. This minimization result is derived from the following two factors: (1) less switching frequency ratio; and (2) the absence of switching in the vicinity of peak input current. As a result, it is anticipated that the switching loss of the rectifier is reduced by 46%, compared with continuous space-vector PWM rectifiers, and 20% compared with conventional discontinuous space-vector PWM rectifiers. Moreover, the proposed PWM scheme can produce the highest available output voltage because it is based on the concept of the voltage space vector. The effectiveness of the proposed PWM strategy is verified by experiments  相似文献   
55.
1. Introduction The requirement of minimal bottom coverageand thick sidewall coverage for PVD-based films forlow via resistance and improved stress migration isnot easy to achieve with traditional depositionmethods. Modern I-PVD techniques give high bot-tom coverage, due to the ionized component of thedeposition flux. Sidewall coverage tends to be low,which is mainly due to off-normal deposition fluxand a less than unity sticking coefficient.  相似文献   
56.
Partial replacement of silver particles by carbon black (low cost) in electrically conductive paint was found to decrease the electrical resistivity and increase the scratch resistance of the resulting thick film, which is for use in electrical interconnections. An effective carbon black content is 0.055 of the total filler volume. By using a total solid volume fraction of 0.1969 and a silane-propanol (1:1 by weight) solution as the vehicle, a paint that gives a thick film with resistivity 2 × 10−3 Ω·cm has been attained.  相似文献   
57.
The design and growth of GaN/InGaN heterojunction bipolar transistors (HBTs) by metalorganic chemical vapor deposition (MOCVD) are studied. Atomic-force microscopy (AFM) images of p+InGaN base layers (∼100 nm) deposited under various growth conditions indicate that the optimal growth temperature is limited to the range between 810 and 830°C due to a trade-off between surface roughness and indium incorporation. At these temperatures, the growth pressure must be kept above 300 Torr in order to keep surface pit density under control. An InGaN graded-composition emitter is adopted in order to reduce the number of V-shaped defects, which appear at the interface between GaN emitter and InGaN base and render an abrupt emitter-base heterojunction nearly impossible. However, the device performance is severely limited by the high p-type base contact resistance due to surface etching damage, which resulted from the emitter mesa etch.  相似文献   
58.
An integrated and new interface circuit with temperature compensation has been developed to enhance the ISFET readout circuit stability. The bridge-type floating source circuit suitable for sensor array processing has been proposed to maintain reliable constant drain-source voltage and constant drain current (CVCC) conditions for measuring the threshold voltage variation of ISFET due to the corresponding hydrogen ion concentration in the buffer solution. The proposed circuitry applied to Si3N4 and Al2O3-gate ISFETs demonstrate a variation of the drain current less than 0.1 μA and drain-source voltage less than 1 mV for the buffer solutions with the pH value changed from 2 to 12. In addition, the scaling circuitry with the VT temperature correction unit (extractor) and LABVIEW software are used to compensate the ISFET thermal characteristics. Experimental results show that the temperature dependence of the Si3N4-gate ISFET sensor improved from 8 mV/°C to less than 0.8 mV/°C.  相似文献   
59.
In this paper, we investigate the effect of water (H2O) molecules evolving from silicon dioxide (SiO2) film deposited by low pressure chemical vapor deposition (LPCVD) at 670 °C on the transistor characteristic of an electrically erasable programmable read only memory (EEPROM) cell. Fourier Transform Infra red (FT-IR) analysis reveals that H2O is captured during film deposition and diffused to silicon surface during high thermal processing. The diffused H2O molecules lower threshold voltage (Vt) of cell transistor and, thus, leakage current of the cell transistor is increased. In erased cell, Vt lowering is 0.25 V in which it increases leakage current of cell transistor from 1 to 100 pA. This results in the lowering of high voltage margin of a 512 Kb EEPROM from 2.8 to 2.6 V at 85 °C.  相似文献   
60.
This letter presents the room-temperature high-frequency operation of Si/SiGe-based resonant interband tunnel diodes that were fabricated by low-temperature molecular beam epitaxy. The resulting devices show a resistive cutoff frequency f/sub r0/ of 20.2 GHz with a peak current density of 218 kA/cm/sup 2/, a speed index of 35.9 mV/ps, and a peak-to-valley current ratio of 1.47. A specific contact resistivity of 5.3/spl times/10/sup -7/ /spl Omega//spl middot/cm/sup 2/ extracted from RF measurements was achieved by Ni silicidation through a P /spl delta/-doped quantum well by rapid thermal sintering at 430/spl deg/C for 30 s. The resulting devices are very good candidates for RF high-power mixed-signal applications. The device structures presented here are compatible with a standard complementary metal-oxide-semiconductor or heterojunction bipolar transistor process.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号