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   55332篇
  免费   5617篇
  国内免费   35篇
  2021年   697篇
  2020年   435篇
  2019年   526篇
  2018年   683篇
  2017年   632篇
  2016年   1142篇
  2015年   1925篇
  2014年   2070篇
  2013年   2655篇
  2012年   3318篇
  2011年   3354篇
  2010年   2202篇
  2009年   2103篇
  2008年   3032篇
  2007年   3026篇
  2006年   2951篇
  2005年   2913篇
  2004年   2819篇
  2003年   2798篇
  2002年   2724篇
  2001年   857篇
  2000年   759篇
  1999年   865篇
  1998年   778篇
  1997年   602篇
  1996年   557篇
  1995年   538篇
  1994年   538篇
  1993年   547篇
  1992年   603篇
  1991年   551篇
  1990年   575篇
  1989年   473篇
  1988年   516篇
  1987年   492篇
  1986年   442篇
  1985年   547篇
  1984年   548篇
  1983年   484篇
  1982年   583篇
  1981年   569篇
  1980年   525篇
  1979年   437篇
  1978年   431篇
  1977年   390篇
  1976年   388篇
  1975年   316篇
  1974年   409篇
  1973年   384篇
  1972年   269篇
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
1.
2.
China has recently made available hourly air pollution data from over 1500 sites, including airborne particulate matter (PM), SO2, NO2, and O3. We apply Kriging interpolation to four months of data to derive pollution maps for eastern China. Consistent with prior findings, the greatest pollution occurs in the east, but significant levels are widespread across northern and central China and are not limited to major cities or geologic basins. Sources of pollution are widespread, but are particularly intense in a northeast corridor that extends from near Shanghai to north of Beijing. During our analysis period, 92% of the population of China experienced >120 hours of unhealthy air (US EPA standard), and 38% experienced average concentrations that were unhealthy. China’s population-weighted average exposure to PM2.5 was 52 μg/m3. The observed air pollution is calculated to contribute to 1.6 million deaths/year in China [0.7–2.2 million deaths/year at 95% confidence], roughly 17% of all deaths in China.  相似文献   
3.
Birth seasonally at high latitudes is a complex phenomenon which is undoubtedly affected by a subtle interaction between environmental rhythmicity (most notably in photoperiod and temperature) and cultural adaption. There is intriguing evidence that human gonadotrophic activity (and hence fertility) may be affected by seasonal fluctuations in light intensity and duration. Nevertheless, cultural factors are important insofar as they mediate between environmental rhythmicity and human fertility/birth patterns. This article examines the distribution of births over several decades in an Inuit community located 300 miles north of the Arctic Circle. Several shifts in birth seasonality are noted, the most significant of which is a dramatic shift from pronounced seasonality in the 1970s to non-seasonality in the 1980s. Longitudinal ethnographic fieldwork has allowed an examination of social and economic changes accounting for the rather sudden disappearance of birth seasonality. These include increasing reliance upon wage employment and social assistance, decreased dependence upon subsistence hunting and trapping, changing attitudes on the part of young people entering their prime reproductive years, and the introduction of television, radio, and southern-style recreational activities.  相似文献   
4.
We propose a novel hybrid single-electron device for reprogrammable low-power logic operations, the magnetic single-electron transistor (MSET). The device consists of an aluminium single-electron transistor with a GaMnAs magnetic back-gate. Changing between different logic gate functions is realized by reorienting the magnetic moments of the magnetic layer, which induces a voltage shift on the Coulomb blockade oscillations of the MSET. We show that we can arbitrarily reprogram the function of the device from an n-type SET for in-plane magnetization of the GaMnAs layer to p-type SET for out-of-plane magnetization orientation. Moreover, we demonstrate a set of reprogrammable Boolean gates and its logical complement at the single device level. Finally, we propose two sets of reconfigurable binary gates using combinations of two MSETs in a pull-down network.  相似文献   
5.
The ubiquitin-proteasome system for protein degradation plays a major role in regulating cell function and many signaling proteins are tightly controlled by this mechanism. Among these, Regulator of G Protein Signaling 2 (RGS2) is a target for rapid proteasomal degradation, however, the specific enzymes involved are not known. Using a genomic siRNA screening approach, we identified a novel E3 ligase complex containing cullin 4B (CUL4B), DNA damage binding protein 1 (DDB1) and F-box protein 44 (FBXO44) that mediates RGS2 protein degradation. While the more typical F-box partners CUL1 and Skp1 can bind FBXO44, that E3 ligase complex does not bind RGS2 and is not involved in RGS2 degradation. These observations define an unexpected DDB1/CUL4B-containing FBXO44 E3 ligase complex. Pharmacological targeting of this mechanism provides a novel therapeutic approach to hypertension, anxiety, and other diseases associated with RGS2 dysregulation.  相似文献   
6.
7.
8.
Gelled diet for screwworm (Diptera: Calliphoridae) mass production   总被引:1,自引:0,他引:1  
A rearing system based on a diet gelled with Water-Lock G-400, a synthetic superabsorbent (poly(2-propenamide-co-2-propenoic acid, sodium salt)) (WL), was compared with the standard rearing system (liquid diet suspended in acetate fibers) for the mass production of screwworms, Cochliomyia hominivorax (Coquerel). The WL rearing system yielded 2% heavier pupae, 32% higher egg to pupa survival, and required 54% less diet and 88% less labor than the standard rearing system. Other advantages of the WL system include reduced susceptibility to suboptimal environmental conditions and labor practices and characteristics conducive to centralization and mechanization of rearing procedures.  相似文献   
9.
A polymorphism was identified in the bovine butyrophilin (BTN) gene by digesting poly-merase chain reaction products with the restriction enzyme Hae III. This polymorphism was segregating in a Holstein-Friesian sire selected as part of an ongoing study directed towards the identification of quantitative trait loci affecting milk composition. Screening of a half-sib family established for the heterozygous sire allowed the localization of BTN to bovine chromosome 23 (BTA23).  相似文献   
10.
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