全文获取类型
收费全文 | 4999篇 |
免费 | 148篇 |
国内免费 | 212篇 |
专业分类
系统科学 | 201篇 |
丛书文集 | 123篇 |
教育与普及 | 110篇 |
理论与方法论 | 20篇 |
现状及发展 | 36篇 |
研究方法 | 37篇 |
综合类 | 4831篇 |
自然研究 | 1篇 |
出版年
2024年 | 20篇 |
2023年 | 38篇 |
2022年 | 80篇 |
2021年 | 64篇 |
2020年 | 47篇 |
2019年 | 21篇 |
2018年 | 30篇 |
2017年 | 43篇 |
2016年 | 48篇 |
2015年 | 76篇 |
2014年 | 132篇 |
2013年 | 134篇 |
2012年 | 147篇 |
2011年 | 181篇 |
2010年 | 147篇 |
2009年 | 168篇 |
2008年 | 218篇 |
2007年 | 208篇 |
2006年 | 164篇 |
2005年 | 185篇 |
2004年 | 150篇 |
2003年 | 121篇 |
2002年 | 144篇 |
2001年 | 157篇 |
2000年 | 154篇 |
1999年 | 287篇 |
1998年 | 249篇 |
1997年 | 275篇 |
1996年 | 265篇 |
1995年 | 233篇 |
1994年 | 224篇 |
1993年 | 168篇 |
1992年 | 143篇 |
1991年 | 135篇 |
1990年 | 154篇 |
1989年 | 119篇 |
1988年 | 100篇 |
1987年 | 67篇 |
1986年 | 40篇 |
1985年 | 10篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1970年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有5359条查询结果,搜索用时 781 毫秒
971.
我正在从事<森林培育学>教材的撰写工作.这是国家教委(现教育部)确定的一部重点教材,却是第一次用这个名称,它的前身叫作<造林学>.为什么要作这样的改变,在名词术语应用上确有一段值得深思的变化过程. 相似文献
972.
Tingfang Shen 《科学通报(英文版)》2001,46(21):1842-1845
Based on the analysis of three void damage variety models, this note presents (i) a method that regards the void content as
a damage variation of the grain and (ii) a geometric model for micro-unit of void damage. Equations of the void damage variety
containing void content are analyzed. This work is focused on the measurement of internal damage level and the damage variety
estimation is directly related to the life predication in the practical engineering applications. Nowadays, the critical service
life of the solid grain/polymers is usually presumed at domestic and international level. The strength or strain reduction
of 20% or the stabilizer consumption of 50% is generally regarded as a critical storage life of the solid grain/polymers,
and the service life is predicted by the extrapolation method on Anhenius formula. The applications, however, show that the
above method is unreliable and has significant errors. With the aid of the discontiguous automatic measuring device of real
time volume deformation and void content, the master curve of void damage variety, the storage service life and the surplus
life of a solid rocket grain are obtained. Since the critical storage life has been measured, and the accuracy of the service
life prediction has been greatly increased. It is a novel ideal and a break-through technique. 相似文献
973.
Isotopic composition characteristics and identification of immature and lowmature oils 总被引:1,自引:0,他引:1
Isotopic composition characteristics and the significance of immature and low-mature oils are first systematically discussed.
The carbon isotopes of the whole oil can be divided into two groups, one has δ13C main peak values ranging from -30‰ to -29 and the other from -27‰ to-25‰, they are related to lacustrine and salt-lake facies
or swamp facies, respectively. The carbon isotopic fractionation among different group components is relatively small, usually
less than 2‰ and the biggest difference in fractionation often occurs between saturated and aromatic fractions. Their δD values
vary between -180‰ and -130‰ The main peak of their δD values concentrates between -170‰ and -150‰, suggesting a domination
of lacustrine fades. However, the secondary peak ranges from -160‰ to -150‰, showing a frequent salinization of paleo-water
bodies. The average δ13C values of the methane vary between -50‰ and -52‰, about 10‰ lighter than those of mature oils. There is a relatively good
correlationship between immature and low-mature oils and their source rocks in carbon isotopic compositions of group fractions
and monohydrocarbons; moreover, compared with the source rocks of mature oils, that of immature oils is often relatively depleted
in13C, which is one of the characteristics of immature oils, differing from those of mature oils. 相似文献
974.
The stripe disease-specific protein (SP) encoded by the rice stripe virus (RSV) was successfully used as a localization signal
of the virus in its vector, the small brown lanthopper,Laodelphax striatellus Fallen. Immunogold particles in large numbers were detected in various parts of the viruliferous females: the ovum, surface
of chorion, the midgut lumen, and the columnar cells. Whereas there was none of these particles in the non-viruliferous females
and males, and testis of viruliferous males. Endosymbionts (mycetocytes) were abundant, harboring ovaries of both viruliferous
and non-viruliferous females, but none in the testis of males. The results provided us with the direct proof that RSV is a
ciruculative and propagative plant virus and it was transovarially transmitted alongside with endosymbionts of its vector.
Therefore, we deem it a nice lead for future studies on the mechanism of RSV transmission and functioning of its viral proteins. 相似文献
975.
Shen Xianrong Ji Dongmei Hu Youqing Jia Fuxing Deng Xinxian Wang Ling Chu Zhiyong Jiang Dingwen Lei Chengxiang Huang Jiansheng Hu Xinran Ren Daming 《科学通报(英文版)》2001,46(24):2037-2042
Sequences encoding PF4 (58–70) and TSP1 (429–459) were linked to yield a single gene TSF which encodes the fuse-protein of TSF. The gene was cloned into a pGEX-2T expression vector to generate a protein GST-TSF, which was strongly expressed inE. coli. The purified GST-TSF was degraded with thrombin to generate the protein TSF. With the methods of MTT and wound repair assay, the effects of TSF on the proliferation and migration of EC were detected, respectively. The results showed that TSF significantly suppressed BAEC proliferation and migration in a dose-dependent manner. The fuse protein GST-TSF, and the peptides PF4 (58–70) and TSP1 (429–459) also inhibited BAEC proliferation and migration, respectively, but their inhibition rates were not as high as TSF. Using the CAM assay, it was shown that TSF, GST-TSF, PF4 (58–70) and TSP1 (429–459) inhibited angiogenesis in chick CAM potentially, the effect of TSF was the highest.In vivo, the growth of Lewis lung carcinoma was potently inhibited by TSF treatment, and the inhibition rate was 68.75% at a dose of 1.00 μmol/kg · d. These findings suggest that the design on TSF gene was successful, and TSF with its anti-angiogenic and anti-tumor activity, should be a useful source of the inhibitor. 相似文献
976.
The local distribution of preferred orientation and misorientation in TiN whiskers in TiNw-AINp/A1 composite system is investigated using backscattering electron diffraction combined with a scanning electron microscope. These preliminary results demonstrate the utility of this new automated technique for characterizing local textures in composite materials using backscattering electron diffraction. The composite investigated comprises TiN whiskers in a polycrystalline aluminum matrix. The local textures of samples before and after deformation were measured. The misorientation distributions show a breakdown of the monocrystal of TiN whisker in the deformed composite material relative to the starting material and an increase in small angle misorientations. 相似文献
977.
介质阻挡放电照相技术的应用 总被引:5,自引:1,他引:4
在研究介质阻挡放电照相机理和质量的基础上,采用高压纳秒脉冲介质阻挡放电技术,以硬币、植物叶片以及人体手指为样品,对五种应用实例进行了实验尝试和讨论。结果表明,用该方法在拍摄单一色彩的非平面图案时,所得的图像比用普通光学摄影获得的图像更为完整、清晰、且对比度高。该技术可应用于非平面样品图案图像的提取和复印、植物新鲜程度的鉴别、人体指纹的获取以及特殊摄影等领域。 相似文献
978.
彩色等离子体显示器智能接口电路的研究 总被引:1,自引:0,他引:1
提出了可自动识别信号制试和格式的彩色等离子体显示(PDP)接口电路和实现方法,该电路可智能识别并适应常见不同分辨率的VGA信号格式和NTSC、PAL等两种电视制式,其音频处理部分支持最新的丽音标准,带有杜比Pro Logic处理功能。研究和实验结果发现,本接口电路应用于54cm和107cm彩色PDP的图像显示效果均很好,完全达到了实用化的程度。 相似文献
979.
考虑横向剪切效应的竹材层合板弯曲变形 总被引:7,自引:0,他引:7
用层梁经典理论,一阶剪切变形理论以及有限单元法分别计算了竹材层合板梁的弯曲变形,为验证理论分析与数值计算结果是否合理,对实际结构变形进行了测定。结果显示,竹材层合板梁在跨高比较小时具有十分显的的横向剪切交 ,一阶剪切变形理论与有限单元法均能足够精确地反映含有剪切效应的梁的弯曲变形,横向剪切效应很小时,经典理论的解与实际情况的符合程序优于一阶剪切变形理论。 相似文献
980.
Li1+2x +yAlxEuyTi2 -x -ySixP3 -xO12 锂快离子导体可用精选的天然高岭石Al4 [Si4 O10 ](OH) 8为原料 ,与Li2 CO3,TiO2 ,NH4 H2 PO4 ,Eu2 O3 高温 (80 0~ 10 0 0℃ )固相反应约 2 0h而制得 .在 y =0 .5 ,x≤ 0 .3 ;x =0 .2 ,y≤ 0 .3的原始组成范围内 ,可以形成一个空间群为R3C的固熔体相 .它们具有较好的离子电导率和较小的离子导电活化能 .当x =0 .2 ,y =0 .1,t=40 0℃时 ,其电导率达 9.98mS·cm-1,其活化能为 34.0kJ.mol-1. 相似文献