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   3251篇
  免费   79篇
  国内免费   17篇
电工技术   148篇
综合类   9篇
化学工业   755篇
金属工艺   149篇
机械仪表   105篇
建筑科学   61篇
矿业工程   1篇
能源动力   94篇
轻工业   214篇
水利工程   6篇
石油天然气   4篇
无线电   299篇
一般工业技术   514篇
冶金工业   696篇
原子能技术   68篇
自动化技术   224篇
  2023年   10篇
  2022年   21篇
  2021年   63篇
  2020年   19篇
  2019年   22篇
  2018年   49篇
  2017年   23篇
  2016年   52篇
  2015年   47篇
  2014年   75篇
  2013年   146篇
  2012年   108篇
  2011年   177篇
  2010年   104篇
  2009年   142篇
  2008年   156篇
  2007年   131篇
  2006年   90篇
  2005年   113篇
  2004年   106篇
  2003年   92篇
  2002年   82篇
  2001年   90篇
  2000年   64篇
  1999年   79篇
  1998年   234篇
  1997年   177篇
  1996年   130篇
  1995年   79篇
  1994年   80篇
  1993年   80篇
  1992年   39篇
  1991年   40篇
  1990年   34篇
  1989年   35篇
  1988年   30篇
  1987年   31篇
  1986年   14篇
  1985年   21篇
  1984年   19篇
  1983年   10篇
  1982年   31篇
  1981年   23篇
  1980年   29篇
  1979年   36篇
  1978年   14篇
  1977年   25篇
  1976年   39篇
  1975年   11篇
  1974年   5篇
排序方式: 共有3347条查询结果,搜索用时 15 毫秒
11.
12.
A decoupling circuit using an operational amplifier is proposed to suppress substrate crosstalk in mixed-signal system-on-chip (SoC) devices. It overcomes the parasitic inductance problem of on-chip capacitor decoupling. The effect of the proposed decoupling circuit is not limited by parasitic fine impedance. A 0.13-/spl mu/m CMOS test chip showed that substrate noise at frequencies from 40 MHz to 1 GHz was incrementally suppressed by sequentially activating three of the proposed circuits in parallel. The power dissipation of each circuit was 3.3 mW at a 1.0-V power supply. The test chip measurement showed that the proposed decoupling reduced crosstalk by 31% at 200 MHz, whereas it was reduced by 4.4% with capacitor decoupling. This 7:1 ratio, or 17 dB, corresponds to the gain of the opamp. Design of the opamp and its feedback loop for active decoupling is simple, making the opamp useful for SoC applications.  相似文献   
13.
PURPOSE: To report a patient with acute retinal pigment epitheliitis examined less than 24 hours after onset of symptoms. METHOD: One day after the onset of blurred vision in her left eye, a 33-year-old woman had a best-corrected visual acuity of LE, 20/60 -2. The left eye had classic uniform golden-colored nodules in a honeycomb pattern in the foveal retinal pigment epithelium. Intravenous fundus fluorescein angiography disclosed staining of the foveal pigment epithelium. RESULTS: One month after initial examination, visual acuity was LE, 20/20, and fine subfoveal pigmentary clumping was present. CONCLUSION: The pigmentary maculopathy of acute retinal pigment epitheliitis may be nonspecific, resulting from more than one type of primary foveal inflammation.  相似文献   
14.
In this paper, we studied the aging time dependence of the catastrophic optical damage (COD) failure of an Al-free uncoated 0.98-μm GaInAs-GaInP strained quantum-well laser with an injection current as a parameter. Based on the stress-strength model, we first investigated experimentally the dependence of the critical power level (CPL) at which COD would take place upon the aging time. Then applying a statistical treatment to this result, we found for the first time that CPL data at each aging time could be considered to distribute according to the Weibull statistics, and the decrease rate of the CPL with the aging time depended very strongly on the injection current. Finally, using the relationship between the decrease rate of the CPL with the aging time and the current, we predicted roughly the time of a COD failure occurrence for both large and small current cases. As a result, we clarified that for our Al-free uncoated 0.98-μm laser, a COD failure became a fatal problem in the case of a large-current (high-power) operation  相似文献   
15.
OBJECTIVE: To better understand the immunoglobulin variable (V) region repertoire of rheumatoid factors (RF). METHODS: We characterized the heavy (H) and light (L) chain gene segments utilized in a monospecific IgG RF secreting hybridoma (AEE111F) which were derived from a patient with rheumatoid arthritis (RA). The hybridoma was established by fusion of a mouse myeloma cell line with bone marrow derived mononuclear cells from a patient with RA. First strand complementary DNA (cDNA) was generated and used for a polymerase chain reaction amplification of the H and L chain V domains. The amplified V domains were sequenced and compared with an extensive database of germline and cDNA V gene segments. RESULTS: The VH sequence was found to be 96% homologous to a previously described fetal VH3 cDNA (60P2). The VL sequence was also highly homologous to the previously described V lambda II gene (96%) derived from a patient with systemic lupus erythematosus which correlated with an 8.12 idiotype (Id), and to an antibacterial antibody against the Haemophilus influenzae type b capsular polysaccharide (94.7%). CONCLUSION: The overlap among this RF VL gene and the 2 reported V lambda sequences of antibodies that expressed anti-DNA related Id and an environmental pathogen specificity suggests that a part of the IgG RF isolated from patients with RA may thus be derived from the physiological natural antibody repertoire during an abnormal immune response and then develop high affinity, monospecific RF by the selection of an antigen driven mechanism.  相似文献   
16.
Cell death was analyzed in neurulating mouse embryos after in vivo doses of 2-methoxyethanol (2-ME) that produce anterior neural tube defects. Characterization of 2-ME-induced cell death was performed by evaluating: (1) vital fluorochrome staining in whole embryos applying confocal laser scanning microscopy; (2) characteristics of cell debris in conventional histological sections revealed by light microscopy; and (3) Apoptag in situ immunohistochemical staining for apoptosis using light microscopy. Methods for quantification of cell death identified by these three techniques were explored using computerized image analysis. Physiological cell death in control embryos primarily occurred in the neural crest region during neural fold elevation. Embryos exposed to 2-ME had expanded areas of cell death in the neural crest and also new areas of cell death in medial regions of the anterior neural tube. Both physiological and 2-ME-induced embryonic cell death had morphological, immunohistochemical, and fluorochrome staining characteristics of apoptosis. When fluorescence data from confocal microscopic analysis of vital fluorochrome-stained embryos were analyzed, a dose-dependent increase was found in embryos exposed to 2-ME. Similar results were obtained when cell death was analyzed in either conventional histological sections or sections prepared for immunohistochemical detection of apoptosis. The cell death data obtained in this study correlate with previously observed near-term malformation rates, suggesting that a quantitative relationship exists between 2-ME-induced embryonic cell death and neural tube defects.  相似文献   
17.
We have investigated the relation between the crystal structure and superconductivity in La1.9Bi0.1CuO4+δ , in which the phase separation observed in La2CuO4+δ is suppressed. A phase diagram in theT?δ plane is given for La1.9Bi0.1CuO4+δ with excess oxygen. For very smallδ values, the crystal structure is orthorhombic, and an orthorhombic-tetragonal phase transition occurs markedly atδ ~ 0.03 in the measured temperature range between 13 and 293 K. Superconductivity is observed in the range of 0.04<δ<0.11. This is clear evidence thathigh-T c superconductivity also appears in the tetragonal phase.  相似文献   
18.
Malignant human gliomas are the most common forms of primary tumors in the central nerve system. Due to their location and invasive nature, treatment so far has been mainly palliative. Thus, understanding the molecular detail of tumor transformation and progression is crucial for developing effective therapeutic strategy for this fetal tumor. Among the genetic alternations found in these tumors, p53 inactivation and PDGF/PDGFR activation represent the early events, and the loss of chromosome 10 and gene amplification and rearrangement of EGFR represent the late events. Studies with both glioma cell lines and primary tumor tissues have strongly suggested that TGF-alpha and EGFR function as an important autocrine loop in supporting proliferation of human glioma, especially in high grade glioma, since elevated TGF-alpha expression is also found in these high grade tumors. Furthermore, down regulation of the expression of TGF-alpha by antisense constructs has been shown to inhibit several types of human tumor cell growth including glioma. Other means of therapeutic approaches using this autocrine loop as a target also include the use of monoclonal antibodies and their cytotoxic conjugated. Considerable understanding of the EGFR-mediated signal transduction pathways has become available recently, which including GRB2/mSOS1 mediated MAP kinase activation; JAK/STATs pathway; PLC-gamma pathway. However, much work still needs to be done before a specific component of these pathways can be applied for effective control of tumor growth in the clinic.  相似文献   
19.
A case of small epithelioid hemangioendothelioma encapsulated by an egg-shaped bony shell composed of newly formed mature lamellar bone is presented. A nodule measuring 10 x 5 x 5 mm was excised from inside of the right cheek muscle of a 46-year-old man. Histopathologically, the nodule consisted mainly of round-to-short spindled epithelioid cells with round nuclei and occasional cytoplasmic vacuoles associated with some foci of osseous metaplasia. Characteristically, egg shell-like lamellar bone enclosed this lesion showing well-circumscribed appearance. The main component cells were immunohistochemically stained positive for factor VIII-related antigen, CD 34, UEA-1, and vimentin, and proved to be vascular endothelial cells. The term ossifying epithelioid hemangioendothelioma is proposed for this unique lesion. This type of epithelioid hemangioendothelioma has not been previously reported to our knowledge, and differentiation from ossifying fibromyxoid tumor of soft parts is considered to be important.  相似文献   
20.
A solid oxide fuel cell using a thin ceria-based electrolyte film with a Ru-catalyzed anode was directly operated on hydrocarbons, including methane, ethane, and propane, at 600 °C. The role of the Ru catalyst in the anode reaction was to promote the reforming reaction of the unreacted hydrocarbons by the produced steam and CO2, which avoided interference from steam and CO2 in the gas-phase diffusion of the fuels. The resulting peak power density reached 750 mW cm−2 with dry methane, which was comparable to the peak power density of 769 mW cm−2 with wet (2.9 vol.% H2O) hydrogen. More important was the fact that the cell performance was maintained at a high level regardless of the change in the methane utilization from 12 to 46% but was significantly reduced by increasing the hydrogen utilization from 13 to 42%. While the anodic reaction of hydrogen was controlled by the slow gas diffusion, the anodic reaction of methane was not subject to the onset of such a gas-diffusion 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