全文获取类型
收费全文 | 472篇 |
免费 | 16篇 |
国内免费 | 26篇 |
专业分类
系统科学 | 3篇 |
丛书文集 | 21篇 |
教育与普及 | 3篇 |
理论与方法论 | 1篇 |
现状及发展 | 9篇 |
综合类 | 477篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 7篇 |
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 5篇 |
2015年 | 14篇 |
2014年 | 33篇 |
2013年 | 20篇 |
2012年 | 35篇 |
2011年 | 22篇 |
2010年 | 40篇 |
2009年 | 25篇 |
2008年 | 32篇 |
2007年 | 37篇 |
2006年 | 32篇 |
2005年 | 31篇 |
2004年 | 22篇 |
2003年 | 15篇 |
2002年 | 20篇 |
2001年 | 16篇 |
2000年 | 9篇 |
1999年 | 12篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有514条查询结果,搜索用时 0 毫秒
21.
XinMiao Lü LiPing Zhu Mitsugu Nishimura Yoshimune Morita Takahiro Watanabe Toshio Nakamura Yong Wang 《科学通报(英文版)》2011,56(27):2931-2940
A 380-cm-long sediment core was acquired from the deep water area of Pumoyum Co, southern Tibet. Twenty-five plant residue
samples were selected, and organic carbon stable isotopes were obtained using the AMS 14C chronological method. The 14C age and carbon reservoir effect were calibrated with surface sedimentation rate measurements using 210Pb dating. Results showed that the core sediment deposited over 19 cal ka BP. Based on a multi-proxy analysis of TOC and IC
contents, grain size and pollen assemblage data, the palaeoclimatic evolution of Pumoyum Co was reconstructed since the last
glacial. Pumoyum Co was a shallow lake prior to 16.2 cal ka BP; although the glacier around the lake began to melt due to
increasing temperatures, climate was still cold and dry. In the interval of 16.2–11.8 cal ka BP, the sedimentary environment
fluctuated drastically and frequently. Two cold-events occurred at 14.2 and 11.8 cal ka BP, and these may correspond to the
Older Dryas and the Younger Dryas events, respectively. After 11.8 cal ka BP, Pumoyun Co developed into the deep lake as it
is now. The lake water temperature was relatively lower at that time because of influx of cold water from glacial meltwater
entering the lake. As a result, the multi-proxy indicators showed no sign of warm conditions. Comparisons between the sedimentary
record of Pumoyum Co with that of other lakes of the same age in southern Tibet indicate a warmer climate following the last
deglaciation influenced the southeastern Tibetan Plateau. These results imply that the southwest Asian monsoon gradually became
stronger since the deglaciation during its expansion to the inner plateau. The glacial-supplied water of the lake responded
sensitively to cold-events. The entire southern Tibet region was dominantly influenced climatically by the southwest Asian
monsoon during the Holocene. 相似文献
22.
通过基于密度泛函理论(DFT)+U的第一性原理方法研究了单原子Co在TiO2(101)面的掺杂位置和方式、几何结构和整体能量以及掺杂后产物的制氢反应机制,得到了稳定且易出现的单原子修饰结构,即单原子Co吸附在4个O组成的表面空隙的中心位,记为Co/TiO2(101)。进一步对Co/TiO2(101)的析氢反应过程和性能进行研究,确定了当且仅当TiO2(101)面完成表面羟基化反应后,H原子全覆盖的TiO2(101)表面才能进行后续的析氢反应;此时单原子Co是唯一的反应位点,整体的制氢反应自由能ΔGH*比Co(111)面更加趋近于0,显示出其具有远优于金属Co的催化性能。此外,Co和TiO2间的电荷转移和相互作用使TiO2带隙出现新的掺杂能级,可带来作为光催化基材的TiO2光吸收性能的改善。 相似文献
23.
《信阳师范学院学报(自然科学版)》2017,(1):114-117
利用水热法合成了一例新的超分子配合物——5-硝基水杨醛缩乙二胺双希夫碱钴(Ⅲ)配合物.通过元素分析、红外光谱和X-射线单晶衍射等手段对其组成、结构进行了研究,并对合成条件进行了探讨.结果表明,配体采用四齿螯合的方式与金属钴离子配位,配合物结构单元之间通过分子间氢键构筑得到了配合物的超分子网络结构.热重分析表明,配合物具有良好的热稳定性. 相似文献
24.
通过5 wt%NaCl溶液浸泡腐蚀实验和电化学极化实验测量了热浸锌镀层的腐蚀速率和极化曲线,采用光学显微镜、扫描电镜SEM观察了镀层腐蚀后表面的显微组织,初步探讨了腐蚀机理.研究结果表明:锌浴中加入钴的合金镀层抗腐蚀能力要强于纯锌浴镀层;在盐水腐蚀溶液中,锌镀层能够阻滞腐蚀保护基体的主要原因,是牺牲阳极锌和在镀层表面形成了一层由ZnO和ZnCl2.4Zn(OH)2组成的钝化膜;当锌浴中添加适量钴元素后,在镀层中会形成富钴层,可以阻碍镀层的腐蚀,也有利于稳定腐蚀初期生成的Zn(OH)2或ZnCl2.4Zn(OH)2,降低腐蚀电流,提高镀层的耐腐蚀性能. 相似文献
25.
采用溶胶凝胶法,以硝酸钇和柠檬酸为原料对LiNi1/3Co1/3Mn1/3O2进行包覆. 室温下,在2.8~4.3 V和1 C充放电条件下,以柠檬酸协助的Y2O3包覆LiNi1/3Co1/3Mn1/3O2材料作为正极,锂片作为负极,制成的电池50次循环容量没有衰减,而未加柠檬酸的Y2O3包覆LiNi1/3Co1/3Mn1/3O2材料50次循环容量保持率为96.72%,未经过包覆的材料只有91.03%. 相似文献
26.
牛东兰 《科技情报开发与经济》2010,20(10):199-200
利用科技统计对晋城煤业集团在科技创新中的优势和劣势进行了分析,并进一步提出了对策及改进措施。 相似文献
27.
吴锁川 《南京师大学报(自然科学版)》1991,14(4):66-72
本文釆用XRD,XPS,TGA和TPR-XRD技术考察浸渍钴HZSM-5沸石催化剂的还原作用,结果表明:浸渍型载钴沸石经焙烧后,钴主要以Co_3O_40.形态存在于沸石外表面,其粒度为350(?)左右。同时有部分钴离子进入孔内与H~+进行交换,以类似Co(OH)_2形态存在于沸石内表面。500℃还原后,还原度随载钴量增加而增大,其大于57%,粒度在250-270(?)间。还原过程中,出现2个TPR峰,分别相应于过程中的Co~(3+)→Co~(2+)→Co二个阶段,各阶段的特征温度依次为:591-624°K;644-658°K。 相似文献
28.
HUANG Zhiqun XU Zhihong BOYD Sue WILLIAMS David 《科学通报(英文版)》2005,50(22):2581-2586
Decomposition of stumps in successive rotation of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations was studied using a chronosequenee approach. The results showed that decomposition rate constant of Chinese fir stump was 0.02695 as calculated from Olson's model. The N content of stump increased during the first two-year decomposition. When the dead stump C/N ratio was 463.2 ± 27.3, the stumps started to release N. The pattern of P release was similar to that for N. However, K content of stumps showed a consistent declining trend over time during the whole decomposition. ^13C nuclear magnetic resonance spectroscopy with cross polarization and magic-angle spinning (^13C CPMAS-NMR) was used to analyse organic carbon (C) components in decomposing stumps. The ^13C CPMAS-NMR spectra of stumps displayed that stump was dominated by cellulose and hemieellulose. The spectra also showed the accumulation of intensity in alkyl C, aromatic C, and earboxyl C spectral regions, which was expected as the labile cellulose and hemieellulose components in O-alkyl C spectral region were selectively decomposed first. 相似文献
29.
WANG Xichao DAI Bojie CHEN Dayuan LIU Zelong LIU Weimin & DUAN Enkui State Key Laboratory of Reproductive Biology Institute of Zoology Chinese Academy of Sciences Beijing China Correspondence should be addressed to Duan Enkui 《科学通报(英文版)》2003,48(18):1956-1961
Pregnancy between different species is one of the key steps to interspecific somatic cell cloning. Although interspecific clone embryos have been constructed, they could not develop to birth after being transferred to recipi-ents. In order to clarify the mechanism of this phenomenon, interfamily pregnancy between golden hamste (Mesocricetus auratus) and mouse (Mus musculus) was studied. Co-culture results indicated that the adhesion ratios of golden hamster blastocysts on mouse uterine epithelia monolayer 12, 24, 48 and 72 h after co-culture were all significantly lower than those of mouse blastocysts. The outgrowth ratios of golden hamster blastocysts on mouse uterine epithelia monolayer 48, 72 h after co-culture were both significantly lower than those of mouse blastocysts (P < 0.01). Golden hamster抯 blastula could be implanted and develop to D 11 of pregnancy after being transferred to mouse uterus (the 7th day after embryo transfer). Compared to the transfer of mouse embryo to mouse uterus, the successful ratio of interfamily embryo transfer was lower and the bulk of fetus was smaller than that of intraspecific fetus. Compared to intraspecific preg-nancy of mouse, the remote decidual tissue of interfamily pregnancy on D8 is looser. At the same time, expressions of CD57 and CD 68 in remote deciduas were both higher than those in the secondary deciduas in both intraspecific and interfamily pregnancy. However, expressions of the two molecules in interfamily pregnancy were lower than those in intraspecific pregnancy. These results showed that interfam-ily pregnancy could be established between golden hamster and mouse. But the development of fetus in interfamily pregnancy was slower than that in intraspecific pregnancy. The expression difference of CD57 and CD68 indicates the difference of immunoreaction between interfamily and in-traspecific pregnancy, which may be one of the reasons lead-ing to interfamily pregnancy termination. 相似文献
30.
Elemental composition of aerosols collected in the glacier area on Nyainqêntanglha Range, Tibetan Plateau, during summer monsoon season 总被引:1,自引:0,他引:1
In order to investigate the elemental composition in atmospheric aerosols and its sources in the glacier area over the Tibetan Plateau (TP), seven totally suspended particle samples were collected continuously at the col of the Zhadang glacier (30°28′N,90°39′E,5800 m a.s.l.), Nyainqêntanglha Range, southern TP, from June to October 2006. Twenty-seven elements (Li, Be, B, Na, Mg, Al, K, Ca, Sc, Ti, V, Fe, Mn, Zn, Ga, As, Rb, Sr, Y, Cd, Cs, Ba, Tl, Pb, Bi, Th, U) were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The result indicates that the concentrations of most elements (especially crustal elements) are lower than values at the Nam Co Station during the same period of 2005, and also much lower than other sites in the TP such as Wudaoliang and Waliguan. This suggests that elemental compositions of aerosols in the Zhadang glacier area may represent the background levels of the middle/upper troposphere over the TP. Crustal enrichment factors (EFs) reveal that several elements (e.g. B, Zn, As, Cd, Pb and Bi) may have anthropogenic sources. The southern TP is mainly influenced by the summer Indian monsoon during the sampling period. Backward air mass trajectory analysis suggests that air masses in the region may originate from South Asia. Therefore, anthropogenic pollutants from South Asia may be transported by the summer Indian monsoon to the region which clearly affects the atmospheric environment in the southern TP during the summer monsoon season. 相似文献