全文获取类型
收费全文 | 160952篇 |
免费 | 6318篇 |
国内免费 | 3812篇 |
学科分类
工业技术 | 171082篇 |
出版年
2023年 | 1135篇 |
2022年 | 1686篇 |
2021年 | 2907篇 |
2020年 | 2286篇 |
2019年 | 2253篇 |
2018年 | 2896篇 |
2017年 | 3061篇 |
2016年 | 2923篇 |
2015年 | 3223篇 |
2014年 | 4426篇 |
2013年 | 8020篇 |
2012年 | 6154篇 |
2011年 | 7758篇 |
2010年 | 6718篇 |
2009年 | 6933篇 |
2008年 | 7098篇 |
2007年 | 7222篇 |
2006年 | 6821篇 |
2005年 | 6054篇 |
2004年 | 5176篇 |
2003年 | 5118篇 |
2002年 | 4984篇 |
2001年 | 4865篇 |
2000年 | 4073篇 |
1999年 | 4612篇 |
1998年 | 10502篇 |
1997年 | 7081篇 |
1996年 | 5706篇 |
1995年 | 3966篇 |
1994年 | 3392篇 |
1993年 | 3172篇 |
1992年 | 1974篇 |
1991年 | 1851篇 |
1990年 | 1714篇 |
1989年 | 1472篇 |
1988年 | 1308篇 |
1987年 | 928篇 |
1986年 | 941篇 |
1985年 | 946篇 |
1984年 | 826篇 |
1983年 | 713篇 |
1982年 | 716篇 |
1981年 | 683篇 |
1980年 | 579篇 |
1979年 | 468篇 |
1978年 | 408篇 |
1977年 | 532篇 |
1976年 | 953篇 |
1975年 | 295篇 |
1974年 | 271篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
为了探明辽河油田地下水环境中CO2对Q235钢和X70管线钢的腐蚀规律,采用动电位极化技术及交流阻抗技术研究了不同CO2浓度对该环境下两种钢腐蚀行为的影响,并利用光学显微镜对试样腐蚀界面进行表征。结果表明:随着CO2浓度的增加,Q235和X70钢的交流阻抗图谱的半径均减小,腐蚀电流密度增大,腐蚀坑数量、深度和面积都增加,腐蚀敏感性增大。在低浓度CO2环境下(CO2含量为10%时),Q235钢的腐蚀敏感性比X70钢更高;而在高浓度CO2环境下(CO2含量为20%、40%、60%时),Q235钢的腐蚀速率反而较小,耐腐蚀性更好。 相似文献
72.
73.
Plasma Unesterified Fatty‐Acid Profile Is Dramatically and Acutely Changed under Ischemic Stroke in the Mouse Model 下载免费PDF全文
Although plasma biomarkers would facilitate rapid and accurate diagnosis of ischemic stroke for immediate treatment, no such biomarkers have been developed to date. In the present study, we tested our hypothesis that plasma unesterified fatty acids (FFA) are altered at early stages of acute ischemic stroke. Plasma was collected from mice 2 h after the permanent middle cerebral artery occlusion (pMCAo) onset, as well as from sham operated and control animals. After 2 h, pMCAo significantly changed the plasma FFA profile with the most dramatic 2‐ to 3‐fold relative increase in very long n‐3 and n‐6 FFA including 20:4n‐6, 22:4n‐6, 22:5n‐6, and 22:6n‐3. Changes in the plasma FFA profile are consistent with FFA liberation from brain phospholipid hydrolyzed under ischemic insult. These results identify, for the first time, the plasma FFA profile as a potential biomarker for an early ischemic stroke within the therapeutic window for thrombolytic treatment. Further studies are required to confirm its specificity and sensitivity in clinical settings. 相似文献
74.
Guo Liang 《Journal of Modern Optics》2018,65(13):1609-1615
The dynamical properties of spiraling elliptic breathers in nonlocal nonlinear media with linear anisotropy are analytically discussed. Using a two-dimensional asynchronous fractional Fourier transform, the exact analytical solutions of spiraling elliptic breathers are obtained to the nonlocal nonlinear Schrödinger equation with unequal diffraction coefficients in the highly nonlocal limit. It is found that the spiraling elliptic breathers exhibit a kind of molecule-like libration due to the combined effects of the linear anisotropy and the orbital angular momentum. The angular velocity of the spiraling elliptic breathers is discussed, which can be controlled by the linear anisotropy parameter. In the media with linear anisotropy such as uniaxial crystals, the angular velocity of the spiraling elliptic breathers can be controlled by changing the propagation directions of optical beams. Furthermore, the nonlinearity of media is found to enhance the rotation effect of spiraling elliptic breathers. 相似文献
75.
Ming-Puu Chen Asta Y. Z. Lord Yu-Yao Cheng Ku-Chou Tai Wen-Harn Pan 《Journal of Computer Assisted Learning》2020,36(3):383-396
Reflective judgement is crucial for medical-related practitioners in dealing with controversial issues. However, the conformity phenomenon is likely to occur and interfere with reflective judgement learning during interactive activities. Effective strategies are required to moderate the conformity behaviour tendency (CBT) and improve reflective judgement performance (RJP). This study demonstrates two significant results: (a) Compared with the guided self-reflection learning strategy, the online collective reflection (OCR) learning strategy effectively weakened the learners' general CBT while dealing with professional controversial issues; and (b) a significantly negative correlation between the RJP achieved and the change of CBT in online environment was detected in the OCR group. The implications and potential applications in higher education were discussed. Further studies are needed to confirm the long-term effects and the extending application to other professional studies. 相似文献
76.
77.
J. A. Dávila-Pintle E. Reynoso-Lara Y. E. Bravo-García 《Journal of Materials Science: Materials in Electronics》2018,29(18):15358-15362
Samples were grown on glass substrates of CdS doped with different levels of indium via the chemical bath technique. The samples obtained were optically characterized to determine their band-gap. A capacitor of parallel plates is constructed with the CdS samples placed between the plates; afterward, light with energy above the band-gap is used to generate carriers in the CdS to modify its dielectric properties and therefore the capacitance. The sample that causes greater current through the capacitor is used to form part of the tank circuit of a bandpass filter because this tank circuit determines the frequency of resonance or passage of the filter, thus forming a bandpass filter whose resonance frequency depends on the intensity of light that impinges on the CdS sample. Finally, the frequency responses of the filter when the sample is not illuminated compared to that when it is illuminated are studied to demonstrate a filter whose resonance frequency can be effectively controlled by light via the use of indium-doped CdS. 相似文献
78.
Strength of Materials - The development of bimetallic materials is one of the modern lines of research. The differences in the properties of components can impart special mechanical characteristics... 相似文献
79.
80.
Yating Zhang Fuge Niu Xiaoming Zhang Zhengli Lu Yi Guo 《International Journal of Food Properties》2018,21(1):2239-2249
This study aimed to investigate the effect of limited hydrolysis on conformational and antioxidant properties of soy protein isolate-maltodextrin (SPI-Md) conjugates. Extrinsic fluorescence analysis showed unfolding of the protein molecule and exposure of hydrophobic groups in SPI-Md conjugate hydrolysates. Free amino acid analysis showed that, the contents of hydrophobic amino acids in SPI-Md conjugates increased after hydrolysis. The contents of leucine, isoleucine, phenylalanine increased from 0.32, 0.30 and 0.54 to 1.36, 1.86 and 2.60, respectively, when the hydrolysis degree (DH) gradually increased from 0% to 5.7%. The FT-IR spectrum showed that C = O absorption of the amide group formed by glycosylation continued unabated after limited hydrolysis (DH 2.9%). The glycated SPI products showed good reducing power and superior resistance to lipid oxidation (34%, 12 mg mL?1), whereas the limit hydrolysates (DH 2.9%) of SPI-Md conjugates showed more ef?cient radical-scavenging capacity (89.5%, at 12 mg mL?1) and iron-chelation activity (91.3%, at 12 mg mL?1). Results of this study indicated that, slight enzymatic hydrolysis (DH 0–2.9%) could help partially unfolding the globular structure of SPI-Md conjugates without deteriorating amide bonds and had a positive effect on their antioxidant properties. 相似文献