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   401765篇
  免费   20539篇
  国内免费   743篇
林业   26400篇
农学   14758篇
基础科学   3101篇
  53594篇
综合类   67402篇
农作物   18105篇
水产渔业   20652篇
畜牧兽医   183370篇
园艺   6935篇
植物保护   28730篇
  2019年   4003篇
  2018年   5283篇
  2017年   5755篇
  2016年   5767篇
  2015年   4943篇
  2014年   6366篇
  2013年   17167篇
  2012年   10594篇
  2011年   13738篇
  2010年   9557篇
  2009年   9616篇
  2008年   12931篇
  2007年   12296篇
  2006年   12150篇
  2005年   11278篇
  2004年   11065篇
  2003年   10898篇
  2002年   10128篇
  2001年   11418篇
  2000年   11110篇
  1999年   9269篇
  1998年   4772篇
  1997年   4647篇
  1996年   4335篇
  1995年   5234篇
  1994年   4712篇
  1993年   4372篇
  1992年   7620篇
  1991年   8008篇
  1990年   7895篇
  1989年   7840篇
  1988年   7183篇
  1987年   7214篇
  1986年   7288篇
  1985年   7199篇
  1984年   6178篇
  1983年   5612篇
  1982年   4171篇
  1981年   3893篇
  1979年   5752篇
  1978年   4847篇
  1977年   4262篇
  1976年   4056篇
  1975年   4205篇
  1974年   4937篇
  1973年   5037篇
  1972年   4711篇
  1971年   4492篇
  1970年   4355篇
  1969年   4220篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Improvement of both the tensile and impact strength of the same polymeric material has always been a great challenge for the plastic industry. The study focuses on the effect of incorporation of calcium carbonate nanoparticles (0.3 wt% to 15 wt%) into three polypropylene (PP) based matrices viz. PP homopolymer, propylene-ethylene (PP-PE) copolymer and the blend of PP:PP-PE (30:70) to improve their impact behavior without hampering the tensile strength much. A loss in both the tensile and impact properties was observed in PP based nanocomposite. However, PP-PE based nanocomposites showed a significant improvement in impact strength (47 %) at 10 wt% loading with a loss of tensile strength by 22 %. To minimize this loss a blend of PP:PP-PE (30:70) was explored as a matrix. At 10 wt% loading, this matrix showed an improvement of 30 % in impact strength whereas the tensile loss was minimized to 10 %. Further, silane coupling agent which promoted good interfacial adhesion was used for best compositions. The variation of crystalline morphology of the nanocomposites with various formulations was analyzed using differential scanning calorimetry and X-ray diffraction.  相似文献   
73.
74.
Landscape Ecology - The effects of changing land use, and especially urbanisation, on species and functional diversity are of global concern. Eco-estates are a form of urban housing development...  相似文献   
75.
A new cytotoxic thiodepsipeptide, verrucosamide (1), was isolated along with the known, related cyclic peptide thiocoraline, from the extract of a marine-derived actinomycete, a Verrucosispora sp., our strain CNX-026. The new peptide, which is composed of two rare seven-membered 1,4-thiazepane rings, was elucidated by a combination of spectral methods and the absolute configuration was determined by a single X-ray diffraction study. Verrucosamide (1) showed moderate cytotoxicity and selectivity in the NCI 60 cell line bioassay. The most susceptible cell lines were MDA-MB-468 breast carcinoma with an LD50 of 1.26 µM, and COLO 205 colon adenocarcinoma with an LD50 of 1.4 µM. Also isolated along with verrucosamide were three small 3-hydroxy(alkoxy)-quinaldic acid derivatives that appear to be products of the same biosynthetic pathway.  相似文献   
76.
Landscape Ecology - Several case studies investigated the role of ecosystem services in participatory planning processes. However, no systematic study exists that cuts across a large number of...  相似文献   
77.
78.
In a feeding trial lasting for months Halothane-positive fattening pigs were put on a diet containing different concentrations of vitamin E and selenium. After a Myostress application vitamin E and selenium were additionally injected in order to examine stress resistance (CK-test) and regenerative capacity (histological examination of Musculus longissimus dorsi) of skeletal muscle. The study revealed that neither high dietary levels of vitamin E/selenium nor a vitamin E/selenium treatment applied after stress are capable of preventing stress related myopathy nor can they influence its healing up favourably.  相似文献   
79.
Epidermal growth factor (EGF)-like activity was measured in mares' colostrum and milk by radioreceptor assay. Milk samples were collected from 22 mares 1 or more times during early lactation. Samples of colostrum were taken after parturition and before the foal first suckled (presuckle), within 6 hours after the foal first suckled (postsuckle), and on days 1, 2, 4, and 8 of lactation. In the 5 mares from which milk samples were obtained at each sampling time, presuckle colostral mean EGF-like activity (17.8 ng/ml) was greatest (P less than 0.05). The mean values for EGF-like activity at all other sampling times were not significantly different from each other (postsuckle colostrum, 9.7 ng/ml; day 1, 9.6 ng/ml; day 2, 8.5 ng/ml; day 4, 8.0 ng/ml; day 8, 7.8 ng/ml).  相似文献   
80.
In 11 dogs with nasal adenocarcinoma, IV treatment with 2 to 8 cycles of cisplatin at a dosage of 60 mg/m2 of body surface, given at 3-week intervals, resulted in a survival time of 8 to 128 weeks (mean, 32.7 weeks; median, 20 weeks). Radiography revealed complete remission in 2 dogs and partial remission in 1 dog, for an overall response rate of 27%. Clinical problems such as nasal discharge (n = 11), sneezing (n = 6), and epistaxis (n = 4) resolved in 1 to 2 weeks in all dogs after beginning chemotherapy.  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