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   2108篇
  免费   18篇
林业   51篇
农学   56篇
基础科学   3篇
  345篇
综合类   281篇
农作物   254篇
水产渔业   141篇
畜牧兽医   864篇
园艺   25篇
植物保护   106篇
  2022年   24篇
  2021年   38篇
  2020年   24篇
  2019年   32篇
  2018年   49篇
  2017年   46篇
  2016年   47篇
  2015年   45篇
  2014年   60篇
  2013年   91篇
  2012年   103篇
  2011年   136篇
  2010年   64篇
  2009年   57篇
  2008年   96篇
  2007年   108篇
  2006年   68篇
  2005年   79篇
  2004年   71篇
  2003年   75篇
  2002年   51篇
  2001年   33篇
  2000年   43篇
  1999年   37篇
  1998年   11篇
  1997年   20篇
  1996年   16篇
  1994年   14篇
  1992年   23篇
  1991年   33篇
  1990年   31篇
  1989年   27篇
  1988年   20篇
  1987年   28篇
  1986年   27篇
  1985年   27篇
  1984年   20篇
  1983年   19篇
  1979年   20篇
  1978年   11篇
  1977年   19篇
  1976年   11篇
  1974年   17篇
  1973年   23篇
  1972年   18篇
  1971年   14篇
  1970年   18篇
  1969年   19篇
  1968年   12篇
  1966年   11篇
排序方式: 共有2126条查询结果,搜索用时 21 毫秒
171.
Three experiments were undertaken to determine the efficacy of different doses of buparvaquone in the infection and treatment immunization of cattle against Theileria parva derived from African buffalo (Syncerus caffer). Two of these experiments also compared buparvaquone with standard doses of long- and short-acting formulations of oxytetracycline. In addition, different dilutions of stabilates were used in the experiments. In the first experiment, a 10(-1.0) dilution of stabilate was used to infect groups of cattle treated with buparvaquone at doses of between 5 and 0.625 mg kg-1 body weight (bwt) on Day 0 after infection. All control cattle developed severe theileriosis and none of the treatment regimes (including those utilizing long-acting oxytetracycline) prevented the development of theileriosis. Treatment with buparvaquone at 2.5 mg kg-1 bwt or oxytetracycline gave the most satisfactory results. In the second experiment when the sporozoite dose was reduced to 10(-2.0) dilution, buparvaquone treatment at 5 and 2.5 mg kg-1 bwt and short- and long-acting formulations of oxytetracycline reduced reactions greatly. While all the oxytetracycline treated animals produced a serological response and were immune to a 50-fold higher challenge with the immunizing stabilate, several animals in the buparvaquone groups did not show a serological response and were not immune to challenge. In the third experiment, groups of cattle were infected with 10(-1.2), 10(-1.4) and 10(-1.6) dilutions of stabilate and were treated with 2.5 mg kg-1 bwt of buparvaquone. No animals developed severe theileriosis and all seroconverted. On homologous challenge, however, two out of 14 cattle showed severe reactions. It was concluded that further work on immunization using buparvaquone treatment at 2.5 mg kg-1 bwt and 10(-1.6) dilution of the stabilate would have to be carried out before such a system could be used in the field.  相似文献   
172.
The use of regenerated silk fiber is limited due to its inferior mechanical properties in spite of high potential in a wide variety of applications. Many studies have been conducted in order to improve the mechanical properties of the regenerated silk materials, but no one has so far suggested an obvious solution. Meanwhile, some reports showed evidence that structural development of silk protein can be manipulated by physical interactions between silk fibroin (SF) and silk sericin (SS) during the regeneration process, especially in recrystallization process of SF. Such a hypothesis suggests a promising clue to enhance the mechanical properties of silk-based materials. Therefore, in this study, we tried to elucidate how SS can promote developing the molecular chain orientation of SF, resulting in an improvement of mechanical properties of regenerated silk fiber during spinning process. The tensile properties of the regenerated silk fiber were significantly improved compared to those of pure SF fiber when a proper amount of SS was blend with SF; both tenacity and breaking elongation increased by approximately 30 % and 70 % at three fold draw ratio, respectively. Quantitative analysis of X-ray diffraction and Herman’s orientation coefficient confirmed that such an improvement of tensile property was mainly caused by an increase of molecular orientation induced by sericin during the drawing process.  相似文献   
173.
Anthocyanin concentration and composition and the effect of steaming and baking on these were evaluated in tubers of Korean red- and purple-fleshed potato cultivars and breeding clones using liquid chromatography with diode array detection and electrospray ionization-mass spectrometry (LC-DAD-ESI-MS). Twenty-six anthocyanins were isolated, of which 24 were identified. Remarkably, five cis isomers were identified, of which four, viz., cis-petanin, cis-peonanin, petunidin 3-cis-caffeoylrutinoside-5-glucoside, and petunidin 3-cis-feruloylrutinoside-5-glucoside, are reported for the first time. Moreover, pelargonidin 3-p-coumaroylrutinoside-5-glucoside (pelanin), peonidin 3-p-coumaroylrutinoside-5-glucoside (peonanin) and petunidin 3-p-coumaroylrutinoside-5-glucoside (petanin) were identified as the principal anthocyanins. We found that the total anthocyanin content of coloured potatoes was decreased by steaming and baking compared with the raw state. In addition, we performed partial least square discriminant analysis (PLS-DA) to discriminate between the analyzed anthocyanins. Cis isomers seemed to play a vital role as a biomarker in the PLS-DA model based on the type of processing and colour of the tubers.  相似文献   
174.
This review examines the effect of tillage on microbial habitat space, and the roles of microbes in influencing N-transformation processes within a heterogeneous soil environment. Literature relating tillage to microbial processes is assessed critically focusing on (a) degrees of physical disruption and N-processes, (b) interactions between organisms and the soil pore network, and (c) the role of soil structure in mediating oxygen movement to sites of microbial activity in soil. Spatial heterogeneity is shown to be a key characteristic of soil structure and N-transformation processes, impacting on predator:prey relations, microbial habitable pore space, and the modelling of the soil system with respect to denitrification. The latter area is discussed with respect to the notion of how a functional appraisal of soil structure may be approached theoretically, at the aggregate and soil profile scale.  相似文献   
175.
The ocean is a rich resource of flora, fauna, and food. A wild-type bacterial strain showing confluent growth on marine agar with antibacterial activity was isolated from marine water, identified using 16S rDNA sequence analysis as Pseudoalteromonas sp., and designated as strain M2. This strain was found to produce various secondary metabolites including quinolone alkaloids. Using high-resolution mass spectrometry (MS) and nuclear magnetic resonance (NMR) analysis, we identified nine secondary metabolites of 4-hydroxy-2-alkylquinoline (pseudane-III, IV, V, VI, VII, VIII, IX, X, and XI). Additionally, this strain produced two novel, closely related compounds, 2-isopentylqunoline-4-one and 2-(2,3-dimetylbutyl)qunoline-4-(1H)-one, which have not been previously reported from marine bacteria. From the metabolites produced by Pseudoalteromonas sp. M2, 2-(2,3-dimethylbutyl)quinolin-4-one, pseudane-VI, and pseudane-VII inhibited melanin synthesis in Melan-A cells by 23.0%, 28.2%, and 42.7%, respectively, wherein pseudane-VII showed the highest inhibition at 8 µg/mL. The results of this study suggest that liquid chromatography (LC)-MS/MS-based metabolite screening effectively improves the efficiency of novel metabolite discovery. Additionally, these compounds are promising candidates for further bioactivity development.  相似文献   
176.
Propargyl alcohol was coupled to 2,4-hexadiyne-1,6-diol (HDD) and crystallized in the process of ultraviolet irradiation-induced topochemical polymerization. The HDD polymer crystals were used as one component in the fabrication of organic photovoltaic cells, in combination with fullerene as the electron acceptor. The various structures of the produced photovoltaic cells included bilayer, trilayer, and bulk heterojunction structures. Their photovoltaic properties were analyzed in relation to crystal structure, electrochemical properties, and band structure of the HDD polydiacetylene polymers.  相似文献   
177.
178.
A strong interaction between the solar wind and comet Giacobini-Zinner was observed oh 11 September 1985 with the Los Alamos plasma electron experiment on the International Cometary Explorer (ICE) spacecraft. As ICE approached an intercept point 7800 kilometers behind the nucleus from the south and receded to the north, upstream phenomena due to the comet were observed. Periods of enhanced electron heat flux from the comet as well as almost continuous electron density fluctuations were measured. These effects are related to the strong electron heating observed in the cometary interaction region and to cometary ion pickup by the solar wind, respectively. No evidence for a conventional bow shock was found as ICE entered and exited the regions of strongest interaction of the solar wind with the cometary environment. The outer extent of this strong interaction zone was a transition region in which the solar wind plasma was heated, compressed, and slowed. Inside the inner boundary of the transition region was a sheath that enclosed a cold intermediate coma. In the transition region and sheath, small-scale enhancements in density were observed. These density spikes may be due to an instability associated with cometary ion pickup or to the passage of ICE through cometary ray structures. In the center of the cold intermediate coma a narrow, high-density core of plasma, presumably the developing plasma tail was found. In some ways this tail can be compared to the plasma sheet in Earth's magnetotail and to the current sheet in the tail at Venus. This type of configuration is expected in the double-lobe magnetic topology detected at the comet, possibly caused by the theoretically expected draping of the interplanetary magnetic field around its ionosphere.  相似文献   
179.
180.
Polyurethane (PU) copolymer is laterally linked with three kinds of metal ion indicator (calcein, calmagite, or eriochrome black T), with which free metal ion in aqueous solution is intended to be detected by PU color change. Metal ion detection by the indicator-PU fails due to the poor permeation of hydrophilic metal ion into hydrophobic PU layer. Instead, three surfactants with different ionic head groups, aerosol OT (AOT), cetyltrimethylammonium bromide (CTAB), and sodium dodecylsulfate (SDS), are tested for metal ion. Cationic CTAB exhibits an instant PU color change, but anionic AOT and SDS do not respond at all. Reason for the selective detection of cationic surfactant is the complex formation between cationic surfactant and indicator. Molecular interactions between PUs are affected by the laterally linked indicators based on the results by infrared spectra and differential thermal analysis. UV-vis spectra reveal that extra peak arising from the linked indicator appears compared to plain PU. The lateral linking of indicator to PU demonstrates, as well as the selective surfactant detection, a 454 % increase in tensile strength and reproducible shape recovery as high as 99 % compared to plain PU.  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