全文获取类型
收费全文 | 16589篇 |
免费 | 2964篇 |
国内免费 | 2442篇 |
专业分类
化学 | 12329篇 |
晶体学 | 299篇 |
力学 | 986篇 |
综合类 | 177篇 |
数学 | 1798篇 |
物理学 | 6406篇 |
出版年
2024年 | 73篇 |
2023年 | 329篇 |
2022年 | 615篇 |
2021年 | 595篇 |
2020年 | 634篇 |
2019年 | 704篇 |
2018年 | 576篇 |
2017年 | 563篇 |
2016年 | 756篇 |
2015年 | 790篇 |
2014年 | 1006篇 |
2013年 | 1328篇 |
2012年 | 1521篇 |
2011年 | 1670篇 |
2010年 | 1157篇 |
2009年 | 1130篇 |
2008年 | 1147篇 |
2007年 | 1104篇 |
2006年 | 911篇 |
2005年 | 805篇 |
2004年 | 655篇 |
2003年 | 509篇 |
2002年 | 483篇 |
2001年 | 428篇 |
2000年 | 390篇 |
1999年 | 296篇 |
1998年 | 231篇 |
1997年 | 186篇 |
1996年 | 193篇 |
1995年 | 158篇 |
1994年 | 171篇 |
1993年 | 126篇 |
1992年 | 126篇 |
1991年 | 105篇 |
1990年 | 99篇 |
1989年 | 86篇 |
1988年 | 52篇 |
1987年 | 46篇 |
1986年 | 47篇 |
1985年 | 43篇 |
1984年 | 30篇 |
1983年 | 19篇 |
1982年 | 11篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 14篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
941.
考虑爆炸成型弹丸(explosively-formed projectile,EFP)变截面的特性,基于流体力学Bernoulli方程和绝热剪切理论,改进了EFP垂直侵彻装甲钢板靶后破片质量模型,结合已有的试验数据和数值仿真方法检验了改进后模型的准确性。在此基础上,分析了靶板厚度和EFP着靶速度对靶板和EFP产生的靶后破片质量的影响规律。结果表明:相比于改进前的模型,改进后的模型能够更准确地解释靶板和EFP产生的靶后破片质量随靶板厚度和EFP着靶速度的变化规律;当EFP着靶速度为1 650 m/s时,随着靶板厚度从30 mm增大到70 mm,EFP变截面的特性对靶板和EFP产生靶后破片质量的影响不断增强;当靶板厚度为40 mm时,随着EFP着靶速度从1 650 m/s升高到1 860 m/s,EFP变截面的特性对靶板和EFP产生靶后破片质量的影响不断减弱。 相似文献
942.
Chao-Shen Zhang Bei-Bei Zhang Liang Zhong Xiang-Yu Chen Zhi-Xiang Wang 《Chemical science》2022,13(13):3728
A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K3PO4. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K3PO4 greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K3PO4 plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species. 相似文献
943.
Juan Chen Liang Fang Meng Zhang Weixiang Peng Kun Sun Jing Han 《Acta Mechanica Solida Sinica》2021,34(4):506-515
Long-lasting constant loading commonly exists in silicon-based microelectronic con-tact,as well as the chemical mechanical polishing area.In this work,the stres... 相似文献
944.
Photosensitizers that can target and accumulate in mitochondria are expected to achieve good therapeutic effects in photodynamic therapy,as mitochondria are the energy generation factory in cells.Herein,we designed and synthesized a novel mitochondrion-targeting photosensitizer TPC-Py with aggregation-induced emission characteristics for image-guided photodynamic therapy.TPC-Py possessed an efficient production of 1O2,with a quantum yield of 11.65%,upon mild white light irradiation(6 mW/cm2).TPC-Py exhibited good biocompatibility under dark condition,but showed remarkable cytotoxicity towards human cervical carcinoma(HeLa)cells with a half maximal inhibitory concentration(IC50)of 3.2μmol/L when exposed to white light irradiation(14.4 J/cm2).In addition,the Stokes shift of TPC-Py was as high as 150 nm,so that it could prevent self-absorption and increase the signal-to-noise ratio of fluorescence imaging.The excellent performance of TPC-Py makes it a promising candidate in imaging-guided clinical PDT for cancer in the near future. 相似文献
945.
There is a strong correlation between the composition of Deinagkistrodon acutus venom proteins and their potential pharmacological effects. The proteomic analysis revealed 103 proteins identified through label-free proteomics from 30 different snake venom families. Phospholipase A2 (30.0%), snaclec (21.0%), antithrombin (17.8%), thrombin (8.1%) and metalloproteinases (4.2%) were the most abundant proteins. The main toxicity of Deinagkistrodon acutus venom is hematotoxicity and neurotoxicity, and it acts on the lung. Deinagkistrodon acutus venom may have anticoagulant and antithrombotic effects. In summary, the protein profile and related toxicity and pharmacological activity of Deinagkistrodon acutus venom from southwest China were put forward for the first time. In addition, we revealed the relationship between the main toxicity, pharmacological effects, and the protein components of snake venom. 相似文献
946.
Many mushroom species are consumed as food, while significant numbers are also utilised medicinally. Mushrooms are rich in nutrients and bioactive compounds. A growing body of in vitro, in vivo, and human research has revealed their therapeutic potentials, which include such properties as anti-pathogenic, antioxidant, anti-inflammatory, immunomodulatory, gut microbiota enhancement, and angiotensin-converting enzyme 2 specificity. The uses of medicinal mushrooms (MMs) as extracts in nutraceuticals and other functional food and health products are burgeoning. COVID-19 presents an opportunity to consider how, and if, specific MM compounds might be utilised therapeutically to mitigate associated risk factors, reduce disease severity, and support recovery. As vaccines become a mainstay, MMs may have the potential as an adjunct therapy to enhance immunity. In the context of COVID-19, this review explores current research about MMs to identify the key properties claimed to confer health benefits. Considered also are barriers or limitations that may impact general recommendations on MMs as therapy. It is contended that the extraction method used to isolate bioactive compounds must be a primary consideration for efficacious targeting of physiological endpoints. Mushrooms commonly available for culinary use and obtainable as a dietary supplement for medicinal purposes are included in this review. Specific properties related to these mushrooms have been considered due to their potential protective and mediating effects on human exposure to the SARS CoV-2 virus and the ensuing COVID-19 disease processes. 相似文献
947.
In this paper, we propose an improved physical layer key generation scheme that can maximize the secret key capacity by deploying intelligent reflecting surface (IRS) near the legitimate user aiming at improving its signal-to-noise ratio (SNR). We consider the scenario of multiple input single output (MISO) against multiple relevant eavesdroppers. We elaborately design and optimize the reflection coefficient matrix of IRS elements that can improve the legitimate user’s SNR through IRS passive beamforming and deteriorate the channel quality of eavesdroppers at the same time. We first derive the lower bound expression of the achievable key capacity, then solve the optimization problem based on semi-definite relaxation (SDR) and the convex–concave procedure (CCP) to maximize the secret key capacity. Simulation results show that our proposed scheme can significantly improve the secret key capacity and reduce hardware costs compared with other benchmark schemes. 相似文献
948.
949.
C60与亚氨基二乙酸甲酯[NH(CH2COOMe)2]的光化学反应制得2,5-双(甲氧羰基)富勒烯吡咯烷(1), 产率为52% (基于已反应的C60). C60吡咯烷衍生物1与氯乙酸甲酯(ClCH2COOCH3)的N-烃基化反应, 在微波辐射、无溶剂及相转移条件下, 得到2,5-双(甲氧羰基)-N-(甲氧羰基)甲基富勒烯吡咯烷(2), 产率47%(基于C60吡咯烷衍生物1). C60吡咯烷衍生物1和2用NaH, CH3OH水解后, 经盐酸酸化得相应的二羧酸衍生物3和三羧酸衍生物4, 产率分别为65%和53% (基于C60吡咯烷衍生物1和2). C60吡咯烷衍生物1~4的结构由1H NMR, 13C NMR, IR, FAB-MS和元素分析证实. 用电导法测定了C60吡咯烷二羧酸3和三羧酸4钠盐的临界聚集浓度(CAC), 分别为3.58×10-4 mol/L (3)和3.33×10-4 mol/L (4). 这一结果被C60吡咯烷衍生物3和4, 在临界聚集浓度(CAC)附近的UV-Vis光谱的特征变化所支持. 透射电子显微镜(TEM)和静态光散射(SLS)等方法也被用于检测C60吡咯烷衍生物3和4在临界聚集浓度(CAC)时的聚集行为, 结果显示, C60吡咯烷衍生物3在缓冲溶液中(0.001 mol/L NaCO3-NaHCO3), 其聚集体粒径的大小(Rg≈21 nm)不同于C60吡咯烷衍生物4 (Rg≈23 nm). C60吡咯烷二羧酸3钠盐的临界聚集浓度(CAC)比C60吡咯烷三羧酸4钠盐的临界聚集浓度(CAC)大, 聚集体粒径大小的不同, 表明C60单加成衍生物加成基团中羧基(COOH)数目的多少对其聚集行为的影响. 用化学发光法分别检测了C60吡咯烷二羧酸3和C60吡咯烷三羧酸4在缓冲溶液(0.05 mol/L NaCO3-NaHCO3)中对邻苯三酚自氧化产生的超氧阴离子(O2-·)的清除活性. C60衍生物3和4对 O2-· 的清除呈现有明显的剂量效应, 但当超过一定浓度时(3: ~3.50×10-4 mol/L, 4: ~3.25×10-4 mol/L), 清除效率出现转折, 并下降. 这一现象与电导率测定时出现的CAC现象相一致, 进一步证实了C60吡咯烷二羧酸3和C60吡咯烷三羧酸4在较高浓度的水溶液中有聚集的倾向, 也说明了C60吡咯烷二羧酸3和C60吡咯烷三羧酸4的聚集会影响其清除超氧阴离子(O2-·)的活性. 相似文献
950.
采用液相多肽合成法制备得到窄分子量分布、结构可控的生物相容性聚乙二醇嵌段共聚树枝状聚赖氨酸阳离子功能大分子(PEG-b-Dendritic PLL). 运用1H NMR核磁共振、凝胶电泳以及荧光淬灭滴定手段对所得阳离子两嵌段大分子的化学结构及其与质粒DNA (pDNA)结合作用与复合行为进行了研究. 结果表明聚乙二醇嵌段树枝状聚赖氨酸与pDNA分子可以在缓冲溶液中形成稳定的胶束, pDNA与阳离子树枝赖氨酸嵌段通过静电相互作用形成胶束核, 其水溶性聚乙二醇嵌段形成水溶性胶束壳, 提高了阳离子大分子/pDNA复合胶束的稳定性. 同时发现随着阳离子嵌段树枝状赖氨酸代数的增加, 阳离子两嵌段大分子与pDNA的结合作用增强, 有利于其作为基因转染生物功能载体的应用. 相似文献