首页 | 官方网站   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   278415篇
  免费   6063篇
  国内免费   1797篇
工业技术   286275篇
  2021年   2951篇
  2020年   2065篇
  2019年   2484篇
  2018年   4080篇
  2017年   4049篇
  2016年   4464篇
  2015年   3492篇
  2014年   5411篇
  2013年   12905篇
  2012年   8707篇
  2011年   11400篇
  2010年   9189篇
  2009年   9605篇
  2008年   10490篇
  2007年   10597篇
  2006年   9147篇
  2005年   7968篇
  2004年   7114篇
  2003年   6669篇
  2002年   6661篇
  2001年   6675篇
  2000年   6219篇
  1999年   6123篇
  1998年   13061篇
  1997年   9732篇
  1996年   7468篇
  1995年   5711篇
  1994年   5218篇
  1993年   5066篇
  1992年   4060篇
  1991年   3847篇
  1990年   3884篇
  1989年   3796篇
  1988年   3561篇
  1987年   3053篇
  1986年   3073篇
  1985年   3535篇
  1984年   3445篇
  1983年   3137篇
  1982年   2771篇
  1981年   2983篇
  1980年   2714篇
  1979年   2902篇
  1978年   2798篇
  1977年   2894篇
  1976年   3754篇
  1975年   2519篇
  1974年   2343篇
  1973年   2371篇
  1972年   2003篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
Spinel LiSr0·1Cr0·1Mn1·8O4 was synthesised by high temperature solid state method in order to enhance the electrochemical performance. The LiSr0·1Cr0·1Mn1·8O4 (LSCMO) materials were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical tests. The XRD and SEM studies confirm that LSCMO had spinel crystal structure with a space group of Fd3m, and the particle of LSCMO shows irregular shape. The cyclic voltammetry data illustrated that the heavy current charge–discharge performance of LMO was improved by Sr2+ and Cr3+ doping. The galvanostatic charge–discharge of LSCMO cathode materials was measured at 1, 5, 10 and 20 C. The results indicated that LSCMO improved the capacity retention.  相似文献   
93.
Manente  F.  Pietrobon  L.  Ronchin  L.  Vavasori  A. 《Catalysis Letters》2021,151(12):3543-3549
Catalysis Letters - In this work we studied the reactivity of the Trifluoroacetic acid hydroxylamine system in the one step salt free synthesis of amides from ketones. A particular regards was paid...  相似文献   
94.
95.
96.
Russian Journal of Nondestructive Testing - The possibility of approaching the problems of testing the regularities of elastoplastic deformation of 20GL steel by the method of analyzing the...  相似文献   
97.
98.
Colostrum is essential for good neonate health; however, it is not known whether different calves absorb the nutrients from colostrum equally well. In this study, the absorption of protein, IgG, and γ-glutamyl transferase was compared in newborn dairy bull calves for 1 wk after feeding colostrum from different sources. Thirty-five Holstein-Friesian bull calves were randomly allocated into 3 groups and fed colostrum within 4 h after birth. Group A calves (n = 12) were bottle fed colostrum from their own dam for 3 d. Colostrum from these group A cows was also used as foster cow colostrum for the group B calves (n = 12), such that each group A and B calf pair received identical colostrum from each milking of the respective group A dam (10% of birth weight per day). The group C calves (n = 11) were fed 1 bottle (2 L) of pooled colostrum and transition milk (referred to as pooled colostrum), as was the standard practice on the dairy farm. The pooled colostrum was collected from the other dairy cows on the farm 0 to 4 d postpartum and stored at 4°C for less than 12 h. Blood was sampled from calves before the first feeding and at 1, 2, 3, and 7 d after birth. Levels of total solids, total protein, and IgG were higher in the dam colostrum than in the pooled colostrum. At birth, there were no differences between the calf groups for any measurements, and all calves had very low IgG levels. After receiving colostrum, the glucose, plasma γ-glutamyl transferase, serum total protein, and IgG concentrations increased significantly in all calves. There were no differences in any blood measurements at any time point between the pairs of group A and group B calves that received colostrum from the same cow except for the IgG concentration 2 d after birth. However, the group A calves had a higher total serum protein level and IgG concentration than the group C calves for all the time points after the first feeding. The group B calves had a higher IgG concentration than the group C calves on d 1, 2, and 7 after birth. Compared with groups A and B, there was no difference in the proportion of calves in group C that failed to have passive immunity transferred adequately based on the IgG threshold (<10 g/L). Thus, the calves receiving identical colostrum from the same cow had the same levels of IgG, and even the pooled colostrum provided sufficient transfer of IgG as the calves were fed within 4 h after birth.  相似文献   
99.
100.
The objective of this study was to characterise the nutritional potential of leaves and identify a diversity centre with low cyanide and high nutrient content among 178 Latin American cassava genotypes. This field-based collection represents the seven diversity centres, held at The International Center for Tropical Agriculture (CIAT Palmira, Colombia) by the Cassava Program. The cyanide, all-trans-β-carotene and lutein concentrations in cassava leaves ranged from 346 to 7484 ppm dry basis (db), from 174–547 μg g−1 db and 15–181 μg g−1 db, respectively. Cassava leaves also showed significant levels of essential amino acids leucine, lysine, phenylalanine, valine and threonine, and average total protein content of 26.24 g 100 g−1 db. Among seven diversity centres, South American rainforest group showed low cyanide and high carotene content in leaves. In addition, VEN77 and PAN51 genotypes stood out for having low cyanide in leaves and roots and high carotene in leaves. This genetic diversity can be used to select high potential progenitors for breeding purposes.  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-23

京公网安备 11010802026262号