全文获取类型
收费全文 | 15924篇 |
免费 | 355篇 |
国内免费 | 709篇 |
专业分类
系统科学 | 515篇 |
丛书文集 | 542篇 |
教育与普及 | 697篇 |
理论与方法论 | 56篇 |
现状及发展 | 74篇 |
研究方法 | 59篇 |
综合类 | 15044篇 |
自然研究 | 1篇 |
出版年
2024年 | 122篇 |
2023年 | 215篇 |
2022年 | 329篇 |
2021年 | 361篇 |
2020年 | 191篇 |
2019年 | 86篇 |
2018年 | 140篇 |
2017年 | 172篇 |
2016年 | 218篇 |
2015年 | 409篇 |
2014年 | 555篇 |
2013年 | 477篇 |
2012年 | 556篇 |
2011年 | 620篇 |
2010年 | 648篇 |
2009年 | 720篇 |
2008年 | 808篇 |
2007年 | 725篇 |
2006年 | 549篇 |
2005年 | 565篇 |
2004年 | 422篇 |
2003年 | 425篇 |
2002年 | 479篇 |
2001年 | 436篇 |
2000年 | 500篇 |
1999年 | 806篇 |
1998年 | 680篇 |
1997年 | 741篇 |
1996年 | 626篇 |
1995年 | 594篇 |
1994年 | 573篇 |
1993年 | 448篇 |
1992年 | 404篇 |
1991年 | 355篇 |
1990年 | 327篇 |
1989年 | 280篇 |
1988年 | 216篇 |
1987年 | 105篇 |
1986年 | 71篇 |
1985年 | 26篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Correlations for the ignition delay times of hydrogen/air mixtures were developed using the method of High Dimensional Model Representation (HDMR).The hydrogen/air ignition delay times for initial conditions over a wide range of temperatures from 800 to 1600 K,pressures from 0.1 to 100 atm,and equivalence ratios from 0.2 to 10 were first calculated utilizing the full chemical mechanism.Correlations were then developed based on these ignition delay times.Two forms of correlations were constructed:the first one is an overall general model covering the whole range of the initial conditions;while the second one is a piecewise correlation model valid for initial conditions within different sub-domains.The performance of these correlations was studied through comparison with results from the full chemical mechanism as well as experimental data.It was shown that these correlations work well over the whole range of initial conditions and that the accuracy can be significantly improved by using different piecewise correlations for different sub-domains.Therefore,piecewise correlations can be used as an effective replacement for the full mechanism when the prediction of chemical time scale is needed in certain combustion modeling. 相似文献
102.
SCI数据库是国际公认的重要检索系统之一,本文利用汤森路透(ThomsonReu—ters)公司Web of Science中的Science Cita—tion Index Expanded数据库对中国大陆和中国台湾合作的Sa论文进行了全面检索,得到中国大陆SCL论文数量为1413084篇,中国台湾SCI论文数量为332021篇,双方合作SCL论文数量为10480篇。并从论文出版年份分析、5凹类别分析、国家(地区)分析、基金资助机构分析、依托的机构分析、发表期刊的分析等6个方面进行了详细分析,为大家提供了一个关于两岸合作SCL论文的全面分析报告,以期为进一步加强海峡两岸的科学研究方面的合作提供一些有益的线索。 相似文献
103.
如果存在三个v阶拉丁方有fine-structure(t,s),将所有这样的整数对(t,s)的集合记为Fin(v).证明了(2,14),(2,15),(2,16)(≠)Fin(v),v=5,6,7. 相似文献
104.
为了实现核电设施管路的自动化焊缝检测作业,在降低人工作业安全风险的同时提高工作效率,提出了一种集喷涂、喷擦、刷涂、擦洗、环境感知和场景建模的机器人自主作业软件系统及相关算法。分析作业机器人的工作环境和功能需求,进行机械臂和感知模块的选型,制定三维建模和参数辨识方案,搭建焊缝检测作业系统;提出了一种基于机器人感知到的视觉和深度信息进行场景建模的算法,该算法可以输出检测关到作业面的位置、三维模型以及作业区域范围;提出了一种基于三维重构模型和作业区域范围进行作业轨迹规划的算法,并搭建了仿真平台验证运动轨迹的正确性;开展了管道定位与三维重构实验以及轨迹规划实验,实验的结果证明,提出的管道定位方法能够实现精确的点云数据采集和高精度的管道参数辨识,机械臂能够实现准确性高、连续性好的运动,满足管道焊缝检测作业的要求。 相似文献
105.
目前世界能源发展由化石能源向新能源转化,天然气作为清洁的化石能源是两者过渡最好的选择。鄂尔多斯盆地延安地区山西组地层是中国陆相页岩气勘探潜力较大的层系,近年来已经获得了巨大的勘探开发成果。为进一步加深对该地区的地质认识,以鄂尔多斯盆地延安地区山西组泥页岩为研究对象,综合应用野外露头、岩心、测井等资料,开展山西组岩石类型、岩相类型、沉积相展布研究。结果表明:延安地区山西组泥页岩主要发育泥质硅质页岩(S-3)、黏土岩(CM)、含钙/硅质黏土页岩(CM-1)、含钙/黏土质硅质混合页岩(M-2)四种岩相类型。页岩、粉砂质泥岩、碳质泥岩、细粒砂岩4种岩石类型。发育滨浅湖亚相、三角洲前缘亚相和前三角洲亚相,研究区水下分流河道砂体呈南北条带状发育。通过有机质成熟度(Ro)、有机碳含量(TOC)、暗色泥岩厚度将岩相进一步划分为3种类型,其中Ⅰ型岩相为油气勘探有利岩相,在山1段主要分布在延长东部地区、北部地区,山2段主要分布在甘泉下寺湾地区。 相似文献
106.
107.
Zhang Lifan Liu Chong Lian Linsheng Zhao Chunjiang Bao Haigang Wu Changxin 《自然科学进展(英文版)》2007,17(12):1419-1424
Globin protein family plays an important role in storing and transporting oxygen. As a newly reported globin, the revealed function of neuroglobin includes binding and storing oxygen as well as facilitating the utilization of oxygen in neuronal cells. In the present study, mutations in exons of chicken Ngb gene were identified with the method of sequencing and created restriction site PCR in Tibet chicken and other four lowland chicken breeds. The mutations of Lys-2224(E4)-Asn and Ser-2279(E4)-Gly were identified in exon 4 of the gene. The Lys-2224(E4)-Asn mutation existed only in Tibet chicken and the mutation frequencies increased with increasing altitude. Analysis of the haplotype and diplotype of the two mutations in Tibet chicken populations of different altitudes showed that the frequencies of TG haplotype and TTGG diplotype also increased with increasing altitude, while the reverse tendency was found on GGAA diplotype. Under the hypoxic simulation incubation, the main haplotype was TG in living embryos and GA in dead embryos. The results showed that the Lys-2224(E4)-Asn mutation may be a specific mutation associated with hypoxia adaptation in Tibet chicken. 相似文献
108.
为获得眼镜王蛇(Ophiophagus hannah,简称Oh)蛇毒α-神经毒素(α-NT)的基因序列,依据眼镜蛇科不同毒蛇种类来源的α-NT基因有较高的同源性,设计1对上下游引物,为克服引物带来模糊扩增,在蛋白编码部分再设计1对上下游特异引物,用Nacleospin RNA Kit法从3条活眼镜王蛇毒腺中提取mRNA,以3′端引物合成的cDNA作为模板进行PCR扩增反应,测定产物的核苷酸序列,得到全长474bp的眼镜王蛇cDNA基因核苷酸序列。该核苷酸序列的信号肽与眼镜蛇树属Pseudonnaja textilis(Pt)、海蛇Laticauda semifasciata(Ls)100%同源,与眼镜蛇南洋亚种Naja sputatrix (Ns)、银环蛇(Bungarus multicinctus)(Bm)96.8%同源;蛋白密码部分有83.3%与Ns、79.2%与Pt、76.4%与Ls、74.1%与Bm同源。信号肽后紧接着的72个氨基酸有90.3%与已发现的眼镜王蛇毒长链α-NT Toxin a同源,大约有73.6%与Toxin b、69.7%与Oh-4、66.7%与Oh-5、56.9%与Oh-6A和6B同源,并与α-银环蛇毒素54.2%同源。说明新发现的眼镜王蛇cDNA是一条长链α-NT基因。 相似文献
109.
Thawing and freezing processes of active layer in Wudaoliang region of Tibetan Plateau 总被引:8,自引:0,他引:8
The interaction between permafrost and atmosphere is accomplished through transfer of heat and moisture in the overlay active
layer. Thus, the research on the thermal and hydrodynamics of active layer during the thawing and freezing processes was considered
a key to revealing the heat and moisture exchanges between permafrost and atmosphere. The monitoring and research on active
layer were conducted because permafrost occupies about two thirds of the total area of the Tibetan Plateau. Based on the analysis
of the ground temperature data and soil moisture data of monitoring near the Wudaoliang region of the Tibetan Plateau, the
thawing and freezing processes of active layer were divided into four stages, i.e. summer thawing stage (ST), autumn freezing
stage (AF), winter cooling stage (WC) and spring warming stage (SW). Coupled heat and water flow is much more complicated
in ST and AF, and more amount of water is migrating in these two stages. Heat is transferred mainly via conductive heat flow
in the other two stages, and less water migrated. Four water migration and coupled heat flow processes were addressed for
the thawing and freezing stages, which are water infiltration driven by gravity, moisture advection and distillation driven
by temperature and osmotic gradients, water migration driven by capillarity and unfrozen water migration driven by temperature
gradient. The water content near the permafrost table tends to increase after one thawing and freezing cycle, which is the
main reason for the development of thick ground ice layer near permafrost table. 相似文献
110.
Silk fibroin(SF)has been used extensively in the biomedical field including tissue engineering for the generation of artificial bones,skins or ligaments.We have previously reported on good in vitro biocompatibility of SF fibers with peripheral nerve tissues and cells.In the present study,we developed a novel design of the SF-based artificial nerve graft(SF graft)which was composed of a SF-nerve guidance conduit(NGC)inserted with SF fibers.MTT assay was performed to determine the cytotoxicity of the SF-NGC extract fluid on the cultured L929 cells derived from an immortalized mouse fibroblast cell line.In addition,this SF graft was implanted into adult rats for bridging a 10-mm long sciatic nerve defect.The following-up experiments at initial stage(1-4 week)of nerve regeneration including routine blood tests and histochemical investigation were conducted to evaluate the in vivo biocompatibility of the SF graft with peripheral nerves.The results demonstrated that the SF-NGC graft was biocompatible with the surrounding tissues and cells due to its low inflammatory potential with a grade 0 under the U.S.Pharmacopeia guidelines and it was generally suitable to a certain degree for bridging peripheral nerve defects in virtue of supporting Schwann cell adherence,expansion and migration.Therefore the SF graft is a promising alternative to classical autografts for peripheral nerve repair. 相似文献