全文获取类型
收费全文 | 10197篇 |
免费 | 394篇 |
国内免费 | 508篇 |
专业分类
系统科学 | 423篇 |
丛书文集 | 260篇 |
教育与普及 | 196篇 |
理论与方法论 | 31篇 |
现状及发展 | 49篇 |
研究方法 | 46篇 |
综合类 | 10092篇 |
自然研究 | 2篇 |
出版年
2024年 | 65篇 |
2023年 | 134篇 |
2022年 | 176篇 |
2021年 | 204篇 |
2020年 | 145篇 |
2019年 | 74篇 |
2018年 | 94篇 |
2017年 | 127篇 |
2016年 | 136篇 |
2015年 | 252篇 |
2014年 | 322篇 |
2013年 | 288篇 |
2012年 | 333篇 |
2011年 | 405篇 |
2010年 | 395篇 |
2009年 | 431篇 |
2008年 | 483篇 |
2007年 | 571篇 |
2006年 | 429篇 |
2005年 | 508篇 |
2004年 | 325篇 |
2003年 | 242篇 |
2002年 | 276篇 |
2001年 | 311篇 |
2000年 | 305篇 |
1999年 | 492篇 |
1998年 | 458篇 |
1997年 | 498篇 |
1996年 | 437篇 |
1995年 | 347篇 |
1994年 | 318篇 |
1993年 | 275篇 |
1992年 | 283篇 |
1991年 | 224篇 |
1990年 | 224篇 |
1989年 | 185篇 |
1988年 | 142篇 |
1987年 | 112篇 |
1986年 | 59篇 |
1985年 | 9篇 |
1984年 | 1篇 |
1978年 | 2篇 |
1968年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 709 毫秒
341.
Fan Yu Jun Xiao XiangYang Liang ShaoJun Liu GongJian Zhou KaiKun Luo Yun Liu Wei Hu YaPing Wang ZuoYan Zhu 《科学通报(英文版)》2011,56(16):1679-1684
Triploid carp (100%) with 150 (3n=150) chromosomes were obtained by crossing the females of improved tetraploid hybrids (♀,
4n=200) of red crucian carp (♀)×common carp (♂) with the males of diploid yellow river carp (♂, 2n=100). The crosses yielded
transgenic triploid carp (positive triploid fish, 44.2% of the progeny) and non-transgenic triploid carp (negative triploid
fish). Histological examination of the gonads of 24-month-old positive triploid fish suggested they were sterile and the fish
were not able to produce mature gametes during the breeding season. Morphologically, both the positive and negative triploid
fish were similar. They had a spindle-shaped, laterally compressed, steel grey body with two pairs of barbells. Most of the
quantifiable traits of the triploid carp were intermediate between those of the two parents. The positive and negative triploid
fish were raised in the same pond for 2 years. The mean body weight of the positive triploid fish was 2.3 times higher than
the negative triploid fish. The weight of the largest positive triploid fish was 2.91 times higher than that of the largest
negative triploid fish. Thus, we produced fast-growing transgenic triploid carp that have a reduced ecological risk because
of their inability to mate and produce progeny. 相似文献
342.
With the rapid development of wireless communication networks, the fourth generation (4G) has emerged to move beyond the limitations of the third generation (3G), with increased bandwidth, enhanced quality of services, and reduced costs of resources. In this paper, we discuss the key technologies of 4G and focus on surveying the ongoing research in 4G communications in China. The 4G technologies under investigation include multicarrier transmission, multiple antenna techniques, carrier aggregation, relays, cognitive radio, distributed antenna systems, network convergence, and network self-optimization. These technologies are the building blocks of 4G and contribute largely to the requirements of International Mobile Telecommunications Advanced (IMT-Advanced). For each technology, its benefits, research topics, and some existing approaches are examined. Finally, we briefly discuss the challenges and future research issues in 4G systems. 相似文献
343.
Tert-butylhydroquinone-decorated graphene nanosheets and their enhanced capacitive behaviors 总被引:1,自引:0,他引:1
In the present work,tert-butylhydroquinone(TBHQ) was used to decorate graphene nanosheets to obtain a novel and environmentally friendly electrode material for supercapacitors.The fast redox reactions between hydroquinone and quinone generate pseudocapacitance.Graphene layers which have adsorbed TBHQ interact with each other to construct a three-dimensional network.Through this network,electrolyte ions can easily access the surface of graphene to generate electric double-layer capacitance.Electrochemical measurements have shown that using TBHQ as a redox modifier of graphene can obtain a maximum value of 302 F g-1 and provide a 51% enhancement in specific capacitance.Furthermore,excellent rate capability and cycling ability are achieved using the TBHQ-decorated graphene nanosheet electrode. 相似文献
344.
Wang LiBo Yang ZuoSheng ZHang RongPing Fan DeJiang Zhao MeiXun Hu BangQi 《科学通报(英文版)》2011,56(15):1588-1595
Sea surface temperature (SST) records in the South Yellow Sea during the last 6200 years are reconstructed by the unsaturation index of long-chain alkenones (K 37 U ’) in sediment core ZY2 from the central mud area.The SST records varied between 14.1 and 16.5°C (15.6°C on average),with 3 phases:(1) A high SST phase at 6.2-5.9 cal ka BP;(2) A low and intensely fluctuating SST phase at 5.9-2.3 cal ka BP;and (3) A high and stable SST phase since 2.3 cal ka BP.Variation of the SST records is similar to intensity of the Kuroshio Current (KC),and corresponds well in time to global cold climate events.However,the amplitude of the SST response to cooling events was significantly different in different phases.The SST response to global cooling event was weak while the KC was strong;and the SST response was strong while the KC was weak.The difference in amplitude of the SST response is possibly caused by the modulation effect of the Yellow Sea Warm Current which acts as a shelf branch of the KC and a compensating current induced by the East Asia winter monsoon.The warm waters brought by the Yellow Sea Warm Current cushion the SST decrease induced by climate cooling,and both the Kuroshio and East Asian winter monsoon play important roles in the modulation mechanism.The SST records display a periodicity of 1482 years.The same period was found in the KC records,indicating that variation of the SST records in the central South Yellow Sea is strongly affected by KC intensity.The same period was also found in Greenland ice cores and North Atlantic and Arabian Sea sediment cores,showing a regional response of marine environmental variability in the East China Seas to that in the global oceans. 相似文献
345.
The roles of signaling pathways in the production of trypsin proteinase inhibitors (TrypPIs) in rice infested by the leaf folder (LF) Cnaphalocrocis medinalis were studied. Infestation by LF increased TrypPI levels in the leaves of rice plants at the tillering, booting and flowering stages but decreased TrypPI levels at the ripening stage; TrypPI levels in rice stems did not increase at any developmental stage. Infestation by LF at the tillering stage systemically increased TrypPI levels in leaves but not in stems; it also enhanced salicylic acid (SA) levels in leaves and stems, and the ethylene level released from plants. However, LF infestation did not increase JA concentrations. Exogenous application of SA or ethylene enhanced TrypPI levels in the leaves and stems of plants at the tillering stage, whereas treatment with both SA and ethylene induced lower levels of TrypPIs than treatment with SA or ethylene alone, suggesting an antagonistic effect of SA and ethylene on TrypPIs induction. The results suggest that both SA and ethylene signaling pathways are involved in the production of TrypPIs in rice induced by LF; moreover, the antagonistic effect of SA and ethylene may explain the changes in TrypPI levels seen at different plant developmental stages and in different organs. 相似文献
346.
Little is known about the range or controls on the molybdenum isotopic composition of low-medium temperature hydrothermal
ore-forming systems. We present molybdenum isotope data from 12 hydrothermal syndepositional silicalite and carbonaceous slate
samples from the Dajiangping pyrite deposit in western Guangdong Province, South China. The δ
97/95 Mo values from Orebody III range from −0.02‰ to 0.29‰, with an average of 0.18‰. In contrast, the composition values from
Orebody IV display a larger variation from −0.70‰ to 0.62‰. However, the five samples from the main ore bed all show strong
negative values. This indicates that the significant variation in molybdenum isotopic composition supports different hydrothermal
ore-forming metallogenesis and a metallogenic environment between the two orebodies. Orebody III is likely to have been deposited
from submarine exhalative hydrothermal fluids under a relatively strong reducing environment and Orebody IV may have also
been influenced by hydrothermal superimposition in a more oxidized disequilibrium condition. In addition, the δ
97/95 Mo values of Orebody IV are clearly negative, together with the values increasing stratigraphically upward in the ore beds,
suggesting that the metallogenic environment of Orebody IV is different from the open oceanic systems. There might be dynamic
fractionation in this restricted environment. Therefore, the molybdenum isotope can be used as an effective tracer for the
ore-forming fluid and metallogenic environment. 相似文献
347.
传统的矢量量化编码方法总是将待编码矢量以码书中唯一的最匹配码字作为其近似输出矢量,以实现数据压缩的目的.这种方法对远离码字的矢量无法避免显著的误差.本文提出组合编码的矢量量化方法,其思想是对远离码字的矢量进行主辅组合编码,对主码字编码造成的误差通过辅码字加以补偿.实验表明,该方法在很小降低压缩比率的条件下显著提高了矢量编码精度,能够在信号处理等领域发挥有效作用. 相似文献
348.
以鸭梨(Pyrus.bretschneideri Rehd.cv.‘Yali')多倍体(2×、3×、4×)为材料,应用甲基化敏感扩增多态性(methylation sensitive amplified polymorphism,MSAP)技术研究不同倍性鸭梨基因组DNA甲基化水平及模式变化.共筛选22对引物,平均每对引物产生约50条带,统计100~500 bp的扩增带,共计954个位点.分析表明:鸭梨2×、3×、4×分别检测到831、886、852个位点,发生甲基化位点分别占15.5%、20.0%、16.8%,其中全甲基化占9.7%、11.9%、9.7%.分析这些位点甲基化模式表明:甲基化模式在三个倍性间类型相同的占所检测位点的69.2%,仅两种倍性相同的为25.4%,各不相同的占5.5%,其中三倍体41.4%甲基化位点发生改变,显示出独特的DNA甲基化特点. 相似文献
349.
甘肃省降水量时空分布特征浅析 总被引:1,自引:0,他引:1
对甘肃省11个代表性站点1951~2005年降水量系列的分析,发现甘肃省降水量变化呈现出明显的带状分布,且降水量变化具有显著的地域性和时间性。利用距平及累积距平法对甘肃省降水量进行分析,发现在近50年中西北地区降水量较小,但有逐渐增加的趋势,增加幅度较小;南部地区降水量丰富,但有逐渐减小的趋势,降幅较为明显;中部黄土高原地区降水量基本持续在一个相对稳定的水平,且降水量枯、丰水年具有明显的周期变化特征。在甘肃省内部进行局部的南水北调,将对减缓甘肃省北部的用水压力产生很大的作用,使水资源利用更加完善。 相似文献
350.
声速(SOS)是描述声波在媒质中传播特性的一个基本物理量,通过媒质中声速的测定,可以了解媒质的特性或状态变化.基于临界角侧波原理的超声速度测量方法是声速测量方法中的一种,但现有的临界角侧波测量方法精度较低,适用范围小,在测量声速时易受干扰.为此提出了一种新的基于临界角侧波理论的定量超声速度测量方法,该测量方法引入了一个新的参数———倾斜角补偿时间,该参数通过接收时间的差值求得.通过引入该参数,本方法在实际测量中提高了测量精度. 相似文献