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   30643篇
  免费   1057篇
  国内免费   111篇
电工技术   376篇
综合类   229篇
化学工业   5069篇
金属工艺   627篇
机械仪表   584篇
建筑科学   969篇
矿业工程   93篇
能源动力   933篇
轻工业   3859篇
水利工程   241篇
石油天然气   151篇
武器工业   6篇
无线电   2240篇
一般工业技术   4109篇
冶金工业   8368篇
原子能技术   238篇
自动化技术   3719篇
  2023年   150篇
  2022年   491篇
  2021年   782篇
  2020年   531篇
  2019年   598篇
  2018年   687篇
  2017年   767篇
  2016年   717篇
  2015年   551篇
  2014年   850篇
  2013年   1406篇
  2012年   1249篇
  2011年   1576篇
  2010年   1151篇
  2009年   1206篇
  2008年   1095篇
  2007年   960篇
  2006年   770篇
  2005年   834篇
  2004年   903篇
  2003年   811篇
  2002年   786篇
  2001年   690篇
  2000年   538篇
  1999年   597篇
  1998年   2718篇
  1997年   1738篇
  1996年   1161篇
  1995年   723篇
  1994年   566篇
  1993年   649篇
  1992年   233篇
  1991年   299篇
  1990年   238篇
  1989年   211篇
  1988年   239篇
  1987年   177篇
  1986年   180篇
  1985年   193篇
  1984年   128篇
  1983年   93篇
  1982年   124篇
  1981年   128篇
  1980年   130篇
  1979年   92篇
  1978年   78篇
  1977年   202篇
  1976年   369篇
  1975年   87篇
  1973年   51篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
61.
62.
63.
We evaluated the acute and chronic experimental toxicity of a water extract of saponins from Argania spinosa following oral and intraperitoneal (i.p.) administration in mice (Iops Ofa) and rats (Wistar). The DL50 obtained were 79 mg/kg for the i.p. route and 1,300 mg/kg for the oral route. For the chronic toxicity studies, we administred 100 and 200 mg/kg orally once a day during a 3 month period. There was a decrease in blood sugar in the third month of each therapy. Blood creatinine levels increased, thus evoking a renal pathology. A slight increase in transaminases levels was not significatif. Hematologic parameters were unchanged during the treatment and the histopathologic study showed hepatic glycogen decrease and a focal renal tube deterioration.  相似文献   
64.
Flavobacterium aurantiacum NRRL B-184 possesses the ability to degrade aflatoxin B1 in solution and in several food items. Aflatoxin B1 is a potent carcinogen that causes significant economic losses to the agricultural and food industry. The role of trace metal ions (Cu2+, Mn2+, Zn2+, and Co2+) were studied in an effort to understand the enzymatic system involved in aflatoxin B1 degradation by F aurantiacum. The effect of divalent chelators (EDTA and 1,10-phenanthroline [OPT]) in the presence of the trace metal ions was studied as well. Aflatoxin B1 (10 microg/ml) was added to 72-h cultures of F aurantiacum that had been washed and resuspended in phosphate buffer (pH 7.0). HPLC was used to determine aflatoxin B1 concentration in these cultures. Incubating cells at 30 degrees C with 1 and 10 mM Cu2+, Mn2+, and Zn2+ significantly decreased aflatoxin B degradation after 4 and 24 h (P < 0.05). Decreased degradation was also observed with 1 and 10 mM Cu2+ and Zn2+ after 48 h and with 0.1 mM Cu2+ after 24 and 48 h. Co2+ did not have a significant effect on aflatoxin B1 degradation. EDTA and OPT did not counter the inhibition in the presence of Cu2+. The addition of 1 mM EDTA countered the inhibition by 1 mM Mn2+ after 4 and 24 h, but 1 mM OPT did not counter the inhibition by 10 mM Mn2+ after 4 and 24 h. OPT countered the inhibition by 1 mM Zn2+ after 4 and 48 h. These trace elements inhibit aflatoxin B1 degradation by F aurantiacum. In addition, their presence necessitates higher concentrations (>1 mM) of EDTA and OPT for the removal of their inhibitory effect.  相似文献   
65.
Allogeneic blood stem cell transplantation is associated with multiple complications. We report a case of pneumoperitoneum without peritonitis associated with colonic pneumatosis in a patient who had undergone an allogeneic peripheral blood cell transplant  相似文献   
66.
67.
Hybrid cam mechanisms   总被引:10,自引:0,他引:10  
This paper investigates the problem of driving a machine with several reciprocating heavy inertias. The drive should also realize some programmable flexibility of the motion, so that the operation of the machine, e,g., synchronization between different motions and duration of standstills, can easily be modified. The use of a single servomotor for every motion is not the best solution, since it does not allow for energy recuperation. This means that the installed servomotor power will be much higher than the required net peak power for the machine as a whole. The presented solution, however, reduces the servomotor peak torque and power by one order of magnitude, and makes energy transfer between the motions possible. The hybrid cam mechanism consists of a hybrid drive, which is a combination of a servomotor, a constant velocity motor and a cam follower mechanism. The operation principle exploits the nonlinear characteristics of the cam to add flexibility at low cost of energy. The hybrid solution is particularly successful for motions involving high peak acceleration. In addition, the concept can be used to reject disturbances. Simulations show the performance of the hybrid cam mechanism.  相似文献   
68.
69.
de C. Roseno  K. T.  Antunes  R. A.  Alves  R. M. B.  Schmal  M. 《Catalysis Letters》2021,151(12):3639-3655
Catalysis Letters - The NdM0.25Ni0.75O3 (M?=?Cr, Fe) named NCN and NFN catalysts precursors were synthesized and characterized. The CO2 utilization in the feed was studied in the...  相似文献   
70.
Many investigators associate the poor keeping properties of soybean oil with its linolenic acid content. On the other hand the high linoleic acid content is a desired property from a nutritional point of view. We have therefore developed a process for the preferential reduction of the linolenic acid content by selective hydrogenation. Conventional catalysts for the hydrogenation of fats have a rather low selectivity in this respect. When linolenic acid in soybean oil is hardened (e.g., with a nickel catalyst), most of the linoleic acid is converted into less unsaturated acids. It was found that linolenic acid is hydrogenated much more preferentially in the presence of copper catalysts than in that of nickel and other hydrogenation catalysts. At a linolenic acid content of 2%, soybean oil hardened with nickel catalyst contained about 28% linoleic acid, whereas with copper catalyst the hardened soybean oil contained 49% linoleic acid. By means of our process it is possible to manufacture a good keepable oil of, e.g., I.V. 115 and containing 1% linolenic acid and 46% linoleic acid. The storage stability of this product is comparable with that of sunflower-seed oil. A liquid phase yield of 86% is obtained after winterization at 5C for 18 hr. The high selectivity for linolenate reduction of copper catalysts must be ascribed to the copper part of the catalyst. Investigations into the structure of the catalyst indicate that the active center consists of copper metal crystallites; whether these centers are promoted by the carrier or traces of other substances is under investigation.  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