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   636490篇
  免费   8500篇
  国内免费   1914篇
电工技术   12043篇
综合类   878篇
化学工业   95763篇
金属工艺   21911篇
机械仪表   18049篇
建筑科学   17283篇
矿业工程   1577篇
能源动力   17309篇
轻工业   63669篇
水利工程   5151篇
石油天然气   5162篇
武器工业   21篇
无线电   81544篇
一般工业技术   117864篇
冶金工业   120386篇
原子能技术   9432篇
自动化技术   58862篇
  2021年   4442篇
  2019年   4100篇
  2018年   6693篇
  2017年   6596篇
  2016年   6930篇
  2015年   5401篇
  2014年   8996篇
  2013年   29878篇
  2012年   15497篇
  2011年   22012篇
  2010年   17074篇
  2009年   19448篇
  2008年   20713篇
  2007年   20851篇
  2006年   18668篇
  2005年   17286篇
  2004年   16833篇
  2003年   16402篇
  2002年   15892篇
  2001年   16046篇
  2000年   15111篇
  1999年   16019篇
  1998年   37286篇
  1997年   27053篇
  1996年   21324篇
  1995年   16584篇
  1994年   14946篇
  1993年   14327篇
  1992年   10711篇
  1991年   10293篇
  1990年   9832篇
  1989年   9483篇
  1988年   9126篇
  1987年   7668篇
  1986年   7761篇
  1985年   9272篇
  1984年   8557篇
  1983年   7719篇
  1982年   7126篇
  1981年   7194篇
  1980年   6755篇
  1979年   6519篇
  1978年   6093篇
  1977年   7413篇
  1976年   9888篇
  1975年   5225篇
  1974年   5003篇
  1973年   4922篇
  1972年   3972篇
  1971年   3489篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
The MnO activities in (MnO-CrO x -CaO-SiO2)-containing melts, which were saturated with the (Mn, Cr)3O4 spinel phase, were determined at 1500 °C under an oxygen partial pressure of 10−8.99 atm. This was done by equilibrating the samples with platinum. The activity of MnO in the melt was then calculated from the activity coefficient of manganese in the resultant Pt-Cr-Mn alloy. Darken’s quadratic formalism for ternary metallic solutions was used to calculate the activity coefficient of manganese in the Pt-Cr-Mn system, in which platinum was considered to be the solvent. It was found that an increase in the concentration of MnO in the melt increases both the MnO activity and the activity coefficient of MnO. For a constant MnO concentration in the (MnO-CrO x -CaO-SiO2)-containing melts, the activity of MnO can be increased by increasing the basicity of the melt. In order to obtain high-manganese recoveries from (MnO-CrO x -CaO-SiO2)-containing melts into an alloy phase, basic slags in which the activity coefficient of MnO is high should therefore be used.  相似文献   
962.
BACKGROUND AND AIMS OF THE STUDY: Surgical treatment of functional tricuspid regurgitation associated with advanced valvular disease remains controversial, mainly due to the difficulty in choosing between valve replacement or reconstructive surgery. Failure to correct tricuspid regurgitation during valvular surgery carries a poor prognosis, as reoperation may represent a significant risk to the patient. Thus correct tricuspid valve surgery is vital to achieve improved early and long-term clinical results. METHODS: A total of 142 patients underwent concomitant tricuspid annuloplasty with mitral valve replacement and their clinical outcome was assessed. RESULTS: Overall hospital mortality rate was 11.3%. Seven patients died during follow up. The overall actuarial survival rate for 10 years was 74.1 +/- 14.2%. Postoperatively, 75.8% of the patients had no residual tricuspid insufficiency, while 24.2% had first- or second-degree tricuspid insufficiency and were treated medically. All patients were in NHYA functional class I or II postoperatively. CONCLUSION: Clinically, it is important to assess the severity of functional tricuspid insufficiency before and/or during the operation. Bicuspidalization annuloplasty for functional tricuspid insufficiency has provided good results, with a low incidence of reoperation and significant clinical improvement. The technique could be used in most patients with functional tricuspid regurgitation.  相似文献   
963.
964.
PURPOSE: The objectives of this study were to quantify the immunohistochemical stainings of collagen types I, III and IV, and investigate the value of glycohistochemical staining with 3 lectin types specific to a particular glycan structure, Arachis hypogaea, Triticum vulgare and concanavalin A, as a method of defining possible changes in the collagen structure of the tunica albuginea in potent and impotent patients. MATERIALS AND METHODS: The study involved 4 normal men, 4 with pure venous leakage and 4 with pure arterial disease. Collagen types I, III and IV, and lectins Arachis hypogaea, Triticum vulgare and concanavalin A were studied using a cell image processor. The labeling index relates to the percentage of staining and mean optical density relates to the staining intensity. RESULTS: Mean labeling index values for the 3 types of collagen and lectins were similar (p > 0.05). Mean optical density value relating to collagen type I was significantly higher in the arteriogenic group than in the other groups (p < 0.05), while mean optical density value of collagen type IV was significantly higher in the venogenic group than in the 2 other groups (p < 0.05). Mean optical density values relating to the 3 lectin types were similar in the 3 clinical groups (p > 0.05). CONCLUSIONS: An alteration in the distribution and structure of the various collagen types and lectins in the tunica albuginea of impotent patients has been shown that may interfere with normal function and lead to impotence.  相似文献   
965.
The dissociation pK values of the representative loop diuretics furosemide, bumetanide and ethacrynic acid in 10, 30, 40, 50 and 70% (w/w) acetonitrile-water mixtures at 298.15 K were determined, according to the rules and procedures endorsed by IUPAC. The variation in pK values over the whole composition range studied can be explained by tacking into account the preferential solvation of ionizable substances in acetonitrile-water mixtures. With a view to determining the pK values of the loop diuretics studied in any of the binary solvent acetonitrile-water mixtures, correlations of pK values and different bulk properties of the solvent were examined, and the linear solvation energy relationships method, LSER, has been applied. The pK values were then correlated with the pi*, alpha and beta solvatochromic parameters of acetonitrile-water mixtures. The resulting equations allowed us to calculate pK values for the loop diuretics in any acetonitrile-water mixture up to 70% (w/w) acetonitrile.  相似文献   
966.
967.
968.
BACKGROUND: The increase in portal vascular resistance is a significant complication of metastatic disease to the liver or locally advanced cancer, e.g., biliary cancer. PATIENTS AND METHODS: This paper describes the successful palliative treatment of two cancer patients with portal hypertension presenting with the symptoms of tense ascites, mesenteric congestion, and severe variceal bleeding. By creating a stenttract between a hepatic vein and a main branch of the portal vein and/or by placing an extendable stent into the portal vein, the transjugular intrahepatic portosystemic stent-shunt (TIPS) technique was used to decompress the portovascular system. RESULTS: The TIPS-technique offers a new, safe and effective palliation for malignant portal hypertension. In both patients, the symptoms of the portal hypertension disappeared after the procedure. This was accompanied by a significant improvement of the patients performance status allowing an early ambulation. CONCLUSION: Our findings demonstrate the feasibility and effectiveness of the TIPS procedure as a minimal invasive treatment for portal vein decompression in selected tumor patients.  相似文献   
969.
Previous studies have demonstrated that nitric oxide (NO) influences Leydig cell function. Here we provide evidence for NO production and activity in seminiferous tubules and blood vessels of the human testis. By immunohistochemistry, the soluble guanylyl cyclase (sGC), the intracellular NO receptor, and the second messenger, cyclic guanosine monophosphate (cGMP), were detected in myofibroblasts of the peritubular lamina propria in Sertoli cells, as well as in endothelial and smooth muscle cells of testicular blood vessels. Performed with isolated tubules and blood vessels, the biological activity of sGC could be proved by cGMP generation in response to treatments with the NO donor, sodium nitroprusside. The endothelial and neuronal subtypes of NO synthase (NOS) were localized immunohistochemically to the same cell types that express sGC and cGMP. In isolated tubules and vessels, the presence of endothelial NOS and neuronal NOS was confirmed by immunoblotting, and NOS activity was demonstrated by decreased cGMP production upon incubation with the NOS inhibitor L-nitro arginine methylester. These findings show that peritubular cells, Sertoli cells, and testicular blood vessels may be sites of NO production and activity, possibly involved in relaxation of seminiferous tubules and blood vessels to modulate sperm transport and testicular blood flow, respectively.  相似文献   
970.
The G1 glycoprotein of California encephalitis (CE) virus plays a critical role in the infection of mosquito and mammalian cells. We found that CE virus enters baby hamster kidney (BHK-21) and Aedes albopictus (C6/36) cells by the endocytic pathway. Ammonium chloride, a lysosomotropic amine that prevents release of virus from endosomes, inhibited infection of both cell types when added within 10 min after viral adsorption. In addition, infected cells formed polykaryons when the extracellular pH was lowered to 6.3; optimal fusion occurred at pH 5.8 and 6.0 (C6/36 and BHK-21 cells, respectively). Two neutralizing G1 MAba, 6D5.5 and 7D4.5, inhibited low pH-induced syncytia formation without affecting viral attachment, suggesting a role for G1 in viral entry. Since viral fusion proteins have been demonstrated to undergo conformational changes at low pH, acid-induced changes in G1 and G2 were assessed. While both G1 and G2 demonstrated low pH-induced alterations in detergent binding, only G1 displayed an altered protease cleavage pattern at the fusion pH. These results indicate that the G1 protein of CE virus undergoes conformational changes necessary for low pH-mediated entry into both mosquito and mammalian cells.  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