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   129004篇
  免费   4173篇
  国内免费   1429篇
电工技术   2399篇
技术理论   4篇
综合类   3864篇
化学工业   19644篇
金属工艺   6657篇
机械仪表   5404篇
建筑科学   4038篇
矿业工程   1316篇
能源动力   2694篇
轻工业   6753篇
水利工程   1781篇
石油天然气   1361篇
武器工业   146篇
无线电   16726篇
一般工业技术   23742篇
冶金工业   5971篇
原子能技术   794篇
自动化技术   31312篇
  2024年   117篇
  2023年   607篇
  2022年   1077篇
  2021年   1617篇
  2020年   1210篇
  2019年   1111篇
  2018年   15475篇
  2017年   14444篇
  2016年   11075篇
  2015年   2027篇
  2014年   2180篇
  2013年   3100篇
  2012年   5838篇
  2011年   12442篇
  2010年   10614篇
  2009年   8064篇
  2008年   9213篇
  2007年   9906篇
  2006年   2141篇
  2005年   2863篇
  2004年   2608篇
  2003年   2679篇
  2002年   2291篇
  2001年   1551篇
  2000年   1245篇
  1999年   1001篇
  1998年   1376篇
  1997年   941篇
  1996年   793篇
  1995年   604篇
  1994年   507篇
  1993年   429篇
  1992年   320篇
  1991年   295篇
  1990年   273篇
  1989年   258篇
  1988年   209篇
  1987年   192篇
  1986年   150篇
  1985年   157篇
  1984年   126篇
  1983年   82篇
  1982年   77篇
  1981年   69篇
  1980年   84篇
  1978年   64篇
  1977年   61篇
  1976年   81篇
  1955年   71篇
  1954年   69篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
The development of control strategies for loiasis is of crucial importance in endemic areas and depends heavily on the accurate identification of occult-infected individuals. A polymerase chain reaction (PCR) and nested polymerase chain reaction (nested PCR) were developed and based on sequences of the repeat 3 region (15r3) of the gene encoding a Loa loa 15-kD protein. The assays was performed on 20 blood samples from occult-infected subjects and 30 from field-collected amicrofilaremic individuals. The size of the initial PCR product was 396 basepairs (bp). When this initial amplification using primers 15r3(1) and 15r3(2) was carried out for 30 cycles, the PCR products from three of the 20 occult-infected and five of the 30 amicrofilaremic individuals were visualized after electrophoresis by staining the gel with ethidium bromide. Subsequent Southern blotting and hybridization with the specific probe revealed hybridization in 19 of 20 occult-infected and 23 of 30 amicrofilaremic samples but only after two days of exposure of the blot to the x-ray film. When the nested PCR was carried out (product size = 366 bp, primers 15r3(3) and 15r3(4)), 19 of 20 occult-infected and 23 of 30 amicrofilaremic samples that were positive by Southern hybridization of the initial PCR products were strongly positive by staining with ethidium bromide. Qualitative Southern blotting of the nested PCR products using the same probe previously described confirmed the ethidium bromide staining results after a very short exposure time of 4 hr. These results demonstrate that the nested PCR amplification product is specific and that its sensitivity in detecting occult loiasis is 95%. This approach has significant promise for the screening of large human populations for active loiasis without the requirement for blotting and hybridization of the PCR products.  相似文献   
892.
893.
Vibrio parahaemolyticus is an important food-borne enteropathogen that encounters various adverse conditions in its native environment or during infection. Effects of mild acid treatment on survival under stress conditions, enteropathogenicity, and protein production in this pathogen were investigated. Logarithmically grown cells, at pH 7.5 shifted to pH 5.0 for 30 min, were more resistant to subsequent acid challenge at pH 4.4. A two-phase adaptive procedure (pH 5.8 for 30 min; pH 5.0 for 30 min) was better than a single-phase procedure for enhancing the acid tolerance of this pathogen. The acid-adapted cells were cross-protected against the challenges of low salinity and thermal inactivation. One-dimensional polyacrylamide gel electrophoresis revealed that proteins with molecular masses of 6.4, 9.0, 13.6, 16.3, 18.9, 22.9, 24.4, 28.3, 33. 9, 36.9, 41.2, 47.6, 58.1, 65.6, 80.5, 88.2, and 96.9 kDa were induced or significantly enhanced, while proteins of 25.3, 30.1, 30. 7, and 91.7 kDa were significantly inhibited. Two-dimensional polyacrylamide gel electrophoresis revealed that 20 species of proteins were induced or significantly enhanced, while 26 species were inhibited. In assays conducted using the suckling mouse model, enteropathogenicity of the acid-adapted cells was significantly enhanced in terms of intestine/body weight ratio and in vivo recovery of infected cells.  相似文献   
894.
Defects were characterized in epitaxial (001) CeO2 films deposited and planarizedin situ on patterned (001) LaAlO3 substrates by ion beam assisted deposition (IBAD). A hill and valley structure with steps running parallel to the [100] LaAlO3 axis was produced on the surface of the substrate by photolithography and ion beam etching prior to film deposition. A conformai epitaxial CeO2 layer of ∼ 100 nm thickness was deposited on the heated substrate by e-beam evaporation. Lattice-matching between the e-beam film and the substrate was of the type: (001) CeO2∥(001) LaAlO3 and [110] CeO2∥[100] LaAlO3. Evaporative deposition of additional film onto the conformai layer was accompanied by bombardment with a 500 eV argon/oxygen ion beam to promotein situ planarization. Extreme lattice misfit for the orientation (001) CeO2∥(001)LaAlO3 and [001] CeO2∥[001] LaAlO3 caused formation of dislocations in the e-beam CeO2 film in the vicinity of individual ledges in the substrate surface. Coherence of the CeO2 film was locally lost in the step regions of the hill and valley structure. The large patterned steps, which are composed of numerous adjacent ledges in the LaAlO3 surface, caused nucleation of CeO2 with a tilt misalignment of up to ∼5‡ about the substrate [100]. Nucleation and growth of nonepitaxial CeO2 crystallites was observed along the step regions of the film during the IBAD portion of deposition. Defect formation in the e-beam ceria layer due to substrate surface relief indicates that “lattice engineering≓ of multilayer epitaxial structures may not be possible when nonplanar surfaces are created during device fabrication. The IBAD CeO2 layer was more defective than the conformai layer deposited without the impinging ion beam, even in the portions of the film where epitaxy was maintained throughout both layers.  相似文献   
895.
Androgens repress expression of many genes, yet the mechanism of this activity has remained elusive. The cytokine, interleukin-6, is active in a variety of biological systems, and its expression is repressed by androgens. Accordingly we dissected the mechanism of androgen's ability to inhibit interleukin-6 expression at the molecular level. In a series of co-transfection assays, we found that 5alpha-dihydrotestosterone, through the androgen receptor, repressed activation of the interleukin-6 promoter, in part, by inhibiting NFkappaB activity. It did not appear that 5alpha-dihydrotestosterone inhibited NFkappaB by activating the androgen receptor to compete for the NFkappaB response element as we could not detect androgen receptor binding to the IL-6 promoter by DNase I footprinting assay. However, by electrophoretic mobility shift assay we found that 5alpha-dihydrotestosterone repressed formation of NFkappaB middle dotNFkappaB response element complex formation. In LNCaP prostate carcinoma cells, 5alpha-dihydrotestosterone achieved this effect through maintenance of IkappaBalpha protein levels in the face of phorbol ester, a stimulus that results in IkappaBalpha degradation. Finally, we confirmed that IkappaBalpha inhibits NFkappaB-mediated activation of the interleukin-6 promoter. These data suggest that maintenance of IkappaBalpha levels may represent the first identified mechanism for androgen-mediated repression of a natural androgen-regulated gene.  相似文献   
896.
Low cardiac output after open heart operations in neonates and infants carries a high mortality. Delayed sternal closure may be life-saving but may prolong hospital stay and increase costs. To circumvent these issues, we shaped homograft bone and interposed it between the sternal edges to allow primary wound closure in 2 pediatric patients. Midterm results are satisfactory.  相似文献   
897.
Cysteine protease activity in mycelial culture increased 7.7-fold after fruit body formation in Pleurotus ostreatus, using the Leu pNA (LPNA) cleavage assay. The enzyme was purified from fruit bodies and its M(r) was 97,000 by gel filtration and 48,500 by SDS-PAGE, indicating that it is a dimer. The enzyme was sensitive to iodoacetic acid, p-chloromercuribenzoate, N-ethylmaleimide, and HgCl2. The sequence of the first 9 N-terminal amino acids of cysteine protease was ASGLXXAIL.  相似文献   
898.
This article summarizes the results of a combined analysis from two identical multicenter clinical trials that investigated the efficacy and safety of sertraline versus placebo for treating panic disorder. Patients with panic disorder who were treated with sertraline had a statistically significant reduction in the mean number of panic attacks per week (the primary efficacy measure) as compared with placebo (4.8 vs. 2.5, p < .001). Sertraline-treated patients also showed greater improvement that was statistically significant on several ratings of panic disorder symptomatology and functioning. The design characteristics, clinical rating measures, and outcome measures in these trials included most of the features deemed essential by Shear and Maser (1994) in their summary of the NIMH Consensus Conference for the development of standardized assessments for panic disorder. This suggests that the NIMH Consensus Conference played a key role in developing successful multicenter pharmacological treatment studies, such as this one that ultimately demonstrated that sertraline was an effective treatment for panic disorder.  相似文献   
899.
Based on a rearrangement inequality by Hardy, Littlewood, and Polya, we define two-operator algebras for independent random variables. These algebras are called Huffman algebras since the Huffman algorithm on these algebras produces an optimal binary tree that minimizes the weighted lengths of leaves. Many examples of such algebras are given. For the case with random weights of the leaves, we prove the optimality of the tree constructed by the power-of-2 rule, i.e., the Huffman algorithm assuming identical weights, when the weights of the leaves are independent and identically distributed.  相似文献   
900.
Two related ideas for improving the speed of ray-cast volume rendering are studied in this paper. The first is an incremental algorithm for trilinear interpolation, a method commonly used in ray-cast volume rendering to calculate sample values. The incremental algorithm can expedite trilinear interpolation when many samples along a ray are located in one cell. The second is an efficient hybrid volume rendering restricted to parallel projection. In the preprocessing stage, acell template is created to store the information used by the incremental trilinear interpolation. When a cell is parallel projected, the information is retrieved from the template to compute the cell contribution. Because the algorithm with only one template may cause aliasing, an antialiasing technique exploiting multiple cell templates is proposed. With our method, ray-cast volume rendering can be accelerated considerably.  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