全文获取类型
收费全文 | 26226篇 |
免费 | 3241篇 |
国内免费 | 2281篇 |
专业分类
化学 | 13346篇 |
晶体学 | 292篇 |
力学 | 899篇 |
综合类 | 118篇 |
数学 | 2193篇 |
物理学 | 6130篇 |
无线电 | 8770篇 |
出版年
2024年 | 113篇 |
2023年 | 639篇 |
2022年 | 836篇 |
2021年 | 912篇 |
2020年 | 886篇 |
2019年 | 923篇 |
2018年 | 713篇 |
2017年 | 715篇 |
2016年 | 902篇 |
2015年 | 1040篇 |
2014年 | 1181篇 |
2013年 | 1571篇 |
2012年 | 1911篇 |
2011年 | 1914篇 |
2010年 | 1349篇 |
2009年 | 1427篇 |
2008年 | 1605篇 |
2007年 | 1562篇 |
2006年 | 1363篇 |
2005年 | 1242篇 |
2004年 | 1054篇 |
2003年 | 974篇 |
2002年 | 875篇 |
2001年 | 771篇 |
2000年 | 641篇 |
1999年 | 487篇 |
1998年 | 399篇 |
1997年 | 347篇 |
1996年 | 385篇 |
1995年 | 296篇 |
1994年 | 297篇 |
1993年 | 275篇 |
1992年 | 241篇 |
1991年 | 204篇 |
1990年 | 212篇 |
1989年 | 158篇 |
1988年 | 154篇 |
1987年 | 97篇 |
1986年 | 96篇 |
1985年 | 127篇 |
1984年 | 99篇 |
1983年 | 89篇 |
1982年 | 67篇 |
1981年 | 60篇 |
1980年 | 67篇 |
1979年 | 48篇 |
1977年 | 46篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1973年 | 59篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
K.C. Yung C.M.T. Law C.P. Lee B. Cheung T.M. Yue 《Journal of Electronic Materials》2012,41(2):313-321
Sn-3.0Ag-0.5Cu nanosolders were synthesized via a chemical reduction method. Polyvinyl pyrrolidone (PVP) and sodium borohydride
(NaBH4) were employed as surfactant and reducing agent, respectively. Ultraviolet–visible (UV–visible) absorption and x-ray diffraction
patterns revealed that alloying had successfully taken place during the reduction process. Different amounts of PVP and NaBH4 additions influenced the nanosolder particle size. Under varying reaction temperatures and pH values, various ranges of nanosolder
size were obtained. Optimized nanosolders were studied by differential scanning calorimetry to investigate the depression
of the melting temperature, and were analyzed by transmission electron microscopy to measure actual particle sizes. The dependence
of the particle size on the melting temperature was observed. The melting point was depressed to 204.4°C when the average
diameter of the nanosolders was 20 nm. Although SnO2 was formed on the nanosolders, it could be cleaned by citric acid. These low-melting-temperature Sn-Ag-Cu nanosolders are
candidates for use in lead-free interconnect applications. 相似文献
992.
993.
994.
We introduced an easy approach to realize the Laplacian first-order and second-order differential of the object wave in transverse direction by use of one in-line digital hologram. It can be used for pinpoint the position of the objects. 相似文献
995.
Piotr Kozlowski Silvia D. Chang Ran Meng Burkhard Mädler Robert Bell Edward C. Jones S. Larry Goldenberg 《Magnetic resonance imaging》2010
The purpose of this work was to compare diagnostic accuracy of Diffusion Tensor Imaging (DTI), dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) and their combination in diagnosing prostate cancer. Twenty-five patients with clinical suspicion of prostate cancer underwent MRI, prior to transrectal ultrasound-guided biopsies. MRI data were correlated to biopsy results. Logistic regression models were constructed for the DTI parameters, DCE MRI parameters, and their combination. The areas under the receiver operator characteristic curves (AUC) were compared between the models. The nonparametric Wilcoxon signed rank test was used for statistical analysis. The sensitivity and specificity values were respectively 81% (74–87%) and 85% (79–90%) for DTI and 63% (55–70%) and 90% (85–94%) for DCE. The combination “DTI or DCE MRI” had 100% (97–100%) sensitivity and 77% (69–83%) specificity, while “DTI and DCE MRI” had 44% (37–52%) sensitivity and 98% (94–100%) specificity. The AUC for DTI+DCE parameters was significantly higher than that for either DTI (0.96 vs. 0.92, P=.0143) or DCE MRI parameters (0.96 vs. 0.87, P=.00187) alone. In conclusion, the combination of DTI and DCE MRI has significantly better accuracy in prostate cancer diagnosis than either technique alone. 相似文献
996.
997.
Tian Jin Lu Zhao Maolin Huang Yan Yue Zhe-Bin Zheng Won-Hun Ham 《Tetrahedron: Asymmetry》2017,28(5):725-729
Concise stereoselective syntheses of (?)-clavaminol A and deacetyl (+)-clavaminol H have been achieved from simple starting materials. Highlights of the synthesis for (?)-clavaminol A include a highly diastereoselective chelation-controlled hydride reduction of an amino ketone to give the anti amino alcohol directly, and NaBH4-mediated dehalogenation. The main synthetic approach for deacetyl (+)-clavaminol H features a highly diastereoselective chelation-controlled hydride reduction of the amino ketone to construct the anti amino alcohol and a palladium catalyzed hydrogenation reaction at the final step. 相似文献
998.
Yue Shao Hong-Ping Zhao Xi-Qiao Feng Huajian Gao 《Journal of the mechanics and physics of solids》2012,60(8):1400-1419
Studying the structure–property relation of biological materials can not only provide insight into the physical mechanisms underlying their superior properties and functions but also benefit the design and fabrication of advanced biomimetic materials. In this paper, we present a microstructure-based fracture mechanics model to investigate the toughening effect due to the crack-bridging mechanism of platelets. Our theoretical analysis demonstrates the crucial contribution of this mechanism to the high toughness of nacre. It is found that the fracture toughness of nacre exhibits distinct dependence on the sizes of platelets, and the optimized ranges for the thickness and length of platelets required to achieve higher fracture toughness are given. In addition, the effects of such factors as the mechanical properties of the organic phase (or interfaces), the effective elastic modulus of nacre, and the stacking pattern of platelets are also examined. Finally, some guidelines for the biomimetic design of novel materials are proposed based on our theoretical analysis. 相似文献
999.
Some new properties of the Boubaker polynomials expansion scheme are presented in this paper. It is shown in particular that the Boubaker polynomials expansion scheme fits standard Cauchy boundary conditions. 相似文献
1000.