全文获取类型
收费全文 | 459篇 |
免费 | 48篇 |
国内免费 | 72篇 |
专业分类
化学 | 365篇 |
晶体学 | 2篇 |
力学 | 12篇 |
综合类 | 5篇 |
数学 | 66篇 |
物理学 | 129篇 |
出版年
2023年 | 6篇 |
2022年 | 13篇 |
2021年 | 15篇 |
2020年 | 17篇 |
2019年 | 20篇 |
2018年 | 11篇 |
2017年 | 12篇 |
2016年 | 22篇 |
2015年 | 16篇 |
2014年 | 38篇 |
2013年 | 31篇 |
2012年 | 49篇 |
2011年 | 30篇 |
2010年 | 28篇 |
2009年 | 28篇 |
2008年 | 33篇 |
2007年 | 23篇 |
2006年 | 29篇 |
2005年 | 39篇 |
2004年 | 12篇 |
2003年 | 11篇 |
2002年 | 6篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 1篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有579条查询结果,搜索用时 734 毫秒
561.
Composite structure of temperature sensitive chitosan microgel and anomalous behavior in alcohol solutions 总被引:1,自引:0,他引:1
Poly(N-isopropylacrylamide)/chitosan (PNIPAM/CS) core-shell microgel was synthesized by graft copolymerization. The microstructure of copolymers was characterized by FT-IR spectrum and (1)H-nuclear magnetic resonance ((1)H NMR). Transmission electron microscope (TEM) and dynamic light scattering (DLS) measurements display that the microgel has high monodispersity and with a core-shell structure. For swelling the microgel in various alcohol solutions, the particles first shrink; then flocculation occurs resulted from weak aggregation of particles with the increase of alcohol concentration. The investigation of the size of microgels as a function of temperature shows that the thermo-sensitive property is markedly exhibited when the alcohol concentration is low, and vanishes when the alcohol concentration exceeds some value where the microgels have the lowest size. 相似文献
562.
The simple group \(^2F_4(q)\) is proved to be uniquely determined by the set of orders of its maximal abelian subgroups, where \(q=2^{2m+1}\) and \(m\ge 1\) . 相似文献
563.
Xing Gao Weijun Zhong Shue Mei 《The Journal of the Operational Research Society》2014,65(11):1682-1691
This paper investigates information sharing and security investments by two firms provided that their information assets are complementary in the sense that their combined information assets are of significant value, whereas the information asset of a single firm is no value to an attacker. In particular, assuming that each firm chooses its security investment and information sharing individually, we obtain some insights about the optimal choices for the firms and the attacker, which form sharp comparisons with those derived from common (substitutive) firms. We further analyse the effect of a social planner on social total costs by assuming that it can control security investments, information sharing and both of them respectively. We demonstrate that an increase in intervention by the social planner may not necessarily be preferable. 相似文献
564.
Weijun MuDi Wu Jia LiuBing Jia Yan SunWei Su Yaping ZhouLi Zhou 《Applied Surface Science》2011,258(2):950-954
The adsorption of CO2 on metal oxides at ambient temperature received less study largely due to the small adsorption amount. However, the adsorption is of interest in refreshing the atmosphere of isolated spaces. It was shown in the present work that PbO was sensitive to low concentration CO2 in the presence of water. An XPS examination indicated that PbO changed to PbCO3 after the adsorption of CO2; therefore, the adsorption is chemical in nature. In order to enlarge the CO2 capacity, PbO was dispersed on the surface of a silica gel with large surface area (710 m2/g). Both CO2 capacity and adsorption rate indicated that the optimal dispersion manner of PbO is the mono-molecular layer surface coverage. Breakthrough experiments showed that the prepared adsorbent could effectively capture low-concentration CO2 at ambient temperature and pressure yielding a CO2 capacity of 59.1 mg g−1. The saturated adsorbent was regenerated on heating at 380 °C and the CO2 capability was recovered. 相似文献
565.
为了分析不同压实矸石的充填体置换``三下'煤柱所引起的岩层移动及稳定性等问题,
采用弹簧原理和弹塑性理论分析了上覆岩层和矸石充填支撑体的压缩机理, 并推导了矸石充
填体的初期压缩量; 根据充填体的压实原理、岩层力学及相关理论, 推导了不同矸石充填体
压实度所对应的压缩值及等价采高的解析式; 结合工程实例, 采用理论计算和数值模拟方法
分析了不同矸石充填体置换煤柱所对应的上覆岩层移动规律. 结果表明, 随着充填体压实度
的减小, 二次岩层移动所引起的顶板下沉值和等价采高都是不断增大的, 而当孔隙比达到
0.55左右, 所得到的等价采高$H_{e}$增长速度更加灵敏, 并且最大侧向位移增加趋势明显
加快. 相似文献
566.
Zhao YC Yuan J Zhang ZG Xu HG Zheng W 《Dalton transactions (Cambridge, England : 2003)》2011,40(11):2502-2508
Manganese polysulfide cations, MnS(x)(+) (x = 1-10), were studied with mass-selected photodissociation experiments and density functional calculations. We found that MnS(+), MnS(2)(+) and MnS(3)(+) undergo dissociation at 355 nm by loss of S, S(2) and S(3), respectively. The dissociation of larger clusters is relatively complex because of the existence of multiple isomers and multiple dissociation channels. The geometric structures of the low-lying isomers found by theoretical calculations are consistent with the dissociation channels observed in the experiments. The dissociation of MnS(x)(+) clusters occurs mainly by breaking of the Mn-S bonds since they are weaker than the S-S bonds. 相似文献
567.
The iteratively reweighted ? 1 minimization algorithm (IRL1) has been widely used for variable selection, signal reconstruction and image processing. In this paper, we show that any sequence generated by the IRL1 is bounded and any accumulation point is a stationary point of the ? 2–? p minimization problem with 0<p<1. Moreover, the stationary point is a global minimizer and the convergence rate is approximately linear under certain conditions. We derive posteriori error bounds which can be used to construct practical stopping rules for the algorithm. 相似文献
568.
目的 分析胃癌组织特征长链非编码RNA(lncRNA)表达谱,并探讨长链非编码RNA UCA1 在胃癌中的表达及效
应。方法 采用基因芯片检测6 例胃癌患者的肿瘤组织及癌旁正常组织LncRNA表达水平,分析胃癌组织特征lncRNA 表达谱,进一步通过定量PCR 检测40 例胃癌及其对应的癌旁正常组织中UCA1的表达并分析淋巴结转移、肿块大小、细胞分化程度及病理分期等临床病理特征的关系。在此基础上,利用siRNA 下调AGS 胃癌细胞内UCA1 表达水平,利用CCK-8 试剂盒检测UCA1干预对胃癌细胞增殖能力的影响。结果 LncRNA 基因芯片检测到胃癌组织1 021条差异>2倍的lncRNA(P<0.05),定量PCR 验证明显差异表达的UCA1在胃癌组织中的表达显著高于正常组织(P<0.01)。此外,临床病理相关分析显示,UCA1表达水平还与胃癌的细胞分化程度及病理分期相关。分化低的胃癌组织中UCA1表达水平明显高于中、高分化的胃癌组织(P<0.05);病理分期Ⅲ/Ⅳ期胃癌组织的UCA1 表达水平明显高于Ⅰ/Ⅱ期的标本(P<0.05)。此外,下调UCA1 胃癌细胞表达可明显抑制细胞增殖。结论 异常表达的lncRNA 参与了胃癌的发生、发展,其中肿瘤组织上调表达的UCA1是胃癌重要的促癌基因。 相似文献
569.
Ginger, a widely used spice and traditional Chinese medicine, is prone to be contaminated by mycotoxins. A simple, sensitive, and reproducible method based on immunoaffinity column clean‐up coupled with HPLC and on‐line postcolumn photochemical derivatization with fluorescence detection was developed for the simultaneous determination of aflatoxins (AFs) B1, B2, G1, G2, and ochratoxin A (OTA) in 25 batches of gingers and related products marketed in China for the first time. The samples were first extracted by ultrasonication with methanol/water (80:20, v/v) and then cleaned up with immunoaffinity columns for analysis. Under the optimized conditions, the LODs and LOQs for the five mycotoxins were 0.03–0.3 and 0.1–0.9 μg/kg, respectively. The average recoveries ranged from 81.3–100.8% for AFs and from 88.6–99.5% for OTA at three spiking levels. Good linearity was observed for the analytes with correlation coefficients all >0.9995. All moldy gingers were contaminated with at least one kind of the five investigated mycotoxins, while none of them were found in normal gingers. Ginger powder samples were contaminated slightly with the contamination levels below the LOQs, while ginger tea bags were mainly contaminated by OTA at 1.05–1.19 μg/kg and ginger black tea bags were mainly contaminated by AFs at 3.37–5.76 μg/kg. All the contamination levels were below the legally allowable limits. 相似文献
570.