全文获取类型
收费全文 | 4385篇 |
免费 | 920篇 |
国内免费 | 426篇 |
专业分类
化学 | 2987篇 |
晶体学 | 40篇 |
力学 | 295篇 |
综合类 | 55篇 |
数学 | 498篇 |
物理学 | 1856篇 |
出版年
2024年 | 17篇 |
2023年 | 95篇 |
2022年 | 177篇 |
2021年 | 189篇 |
2020年 | 214篇 |
2019年 | 195篇 |
2018年 | 180篇 |
2017年 | 144篇 |
2016年 | 230篇 |
2015年 | 230篇 |
2014年 | 287篇 |
2013年 | 379篇 |
2012年 | 404篇 |
2011年 | 374篇 |
2010年 | 291篇 |
2009年 | 221篇 |
2008年 | 277篇 |
2007年 | 264篇 |
2006年 | 215篇 |
2005年 | 177篇 |
2004年 | 119篇 |
2003年 | 120篇 |
2002年 | 105篇 |
2001年 | 98篇 |
2000年 | 86篇 |
1999年 | 122篇 |
1998年 | 75篇 |
1997年 | 88篇 |
1996年 | 70篇 |
1995年 | 66篇 |
1994年 | 49篇 |
1993年 | 46篇 |
1992年 | 27篇 |
1991年 | 27篇 |
1990年 | 19篇 |
1989年 | 12篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 14篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有5731条查询结果,搜索用时 15 毫秒
61.
YingXinLIU ZuoJunWEI JiXiangCHEN JiYanZHANG XinXueLI XiongHuiWEI 《中国化学快报》2005,16(4):531-533
A novel lanthana-promoted nickel catalyst supported on silica for the liquid phase hydrogenation of m-dinitrobenzene to m-phenylenediamine was prepared by an incipient wetness sequential impregnation method. It was found that Ni-La/SiO2 catalyst exhibited high activity and stability for m-dinitrobenzene hydrogenation. Over this catalyst, the conversion of m-dinitrobenzene and the yield of m-phenylenediamine were up to 97.1% and 93.5%, respectively,at 373 K and 2.6 MPa hydrogen pressure after reaction for 1 h. 相似文献
62.
Furostanol saponins with inhibitory action against COX-2 production from Tupistra chinensis rhizomes
Kun Zou Jun Zhi Wang Jun Wu Yuan Zhou Chuang Liu Fei Jun Dan Ya Xiong Zhang Jin Yang 《中国化学快报》2007,18(10):1239-1242
Two furostanol saponins were obtained from the n-butanol fraction of methanol extract from Tupistra chinensis rhizomes,a folk medicine of Shennongjia Forest District of Hubei Province.Their structures were determined as (25S)-26-O-(β-D-glucopyranosyl)- furost-1β,3β,22α,26-tetrol-3-O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→2)-β-D-glucopyranoside (1) and (25R)- 26-O-(β-D-glucopyranosyl)-furost-1β,3β22α,26-tetrol 3-O-β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→2)-β-D-glu- copyranoside (2),on basis of chemical and spectroscopic evidences.1 and 2 displayed marked inhibitory action towards COX-2 production in macrophages of the rat abdomen induced by LPS at 20μg/mL. 相似文献
63.
High molecular weight copoly(ether ketone)s (PEK) based on 4-(4-hydroxyphenyl)-1-phthalazinone (DHPZ)/4,4′-thiobisphenol (TBP)/4,4′-diflourodiphenylketone (DFK) were prepared by nucleophilic substitution polycondensation. The copolymers were characterized by FT- and ^1H-NMR. Thermal properties and solubility of copolymers were studied. 相似文献
64.
随着城市铁路系统的快速发展,铁路沿线建筑内人群对地铁运行产生噪声的抱怨逐渐增多,为控制室内地铁噪声对人群所产生的负面影响,有必要对地铁噪声产生的多维度负面情绪进行评估。采集了79名受试者对建筑内地铁噪声的多维度负面感受(压抑感,不舒适度和不满意度),基于偏最小二乘法分析了主客观影响因素对多维度负面感受的作用机制,并比较了多维度负面感受在反映地铁噪声影响上的差异。结果表明,多维度负面感受主要取决于主观响度和声学参数,活动干扰度、地铁噪声的敏感度、厌烦度和适应性的影响明显更弱;3个多维度负面感受中,相比于压抑感,不舒适度和不满意度受主观响度的影响轻微更显著,在相同的主观响度感受下,不满意度等级最高。 该文可以为地铁沿线建筑物内噪声负面感受的评价和改善提供参考。 相似文献
65.
Feng Wang Rui Zhao Zheng-Xiong Wang Yue Zhang Zhan-Hong Lin Shi-Jie Liu CFETR Team 《中国物理快报》2021,(5):36-40
Fusion born α particle confinement is one of the most important issues in burning plasmas,such as ITER and CFETR.However,it is extremely complex due to the nonequilibrium characteristics,and multiple temporal and spatial scales coupling with background plasma.A numerical code using particle orbit tracing method(PTC)has been developed to study energetic particle confinement in tokamak plasmas.Both full orbit and drift orbit solvers are implemented to analyze the Larmor radius effects on α particle confinement.The elastic collisions between alpha particles and thermal plasma are calculated by a Monte Carlo method.A triangle mesh in poloidal section is generated for electromagnetic fields expression.Benchmark between PTC and ORBIT has been accomplished for verification.For CFETR burning plasmas,PTC code is used for α particle source and slowing down process calculation in 2D equilibrium.In future work,3D field like toroidal field ripples,Alfven and magnetohydrodynamics instabilities perturbation inducing α particle transport will be analyzed. 相似文献
66.
Wei Zheng Wenxian Ye Pingan Yang Dashuang Wang Yuting Xiong Zhiyong Liu Jindong Qi Yuxin Zhang 《Molecules (Basel, Switzerland)》2022,27(13)
With the rapid development of communication technology in civil and military fields, the problem of electromagnetic radiation pollution caused by the electromagnetic wave becomes particularly prominent and brings great harm. It is urgent to explore efficient electromagnetic wave absorption materials to solve the problem of electromagnetic radiation pollution. Therefore, various absorbing materials have developed rapidly. Among them, iron (Fe) magnetic absorbent particle material with superior magnetic properties, high Snoek’s cut-off frequency, saturation magnetization and Curie temperature, which shows excellent electromagnetic wave loss ability, are kinds of promising absorbing material. However, ferromagnetic particles have the disadvantages of poor impedance matching, easy oxidation, high density, and strong skin effect. In general, the two strategies of morphological structure design and multi-component material composite are utilized to improve the microwave absorption performance of Fe-based magnetic absorbent. Therefore, Fe-based microwave absorbing materials have been widely studied in microwave absorption. In this review, through the summary of the reports on Fe-based electromagnetic absorbing materials in recent years, the research progress of Fe-based absorbing materials is reviewed, and the preparation methods, absorbing properties and absorbing mechanisms of iron-based absorbing materials are discussed in detail from the aspects of different morphologies of Fe and Fe-based composite absorbers. Meanwhile, the future development direction of Fe-based absorbing materials is also prospected, providing a reference for the research and development of efficient electromagnetic wave absorbing materials with strong absorption performance, frequency bandwidth, light weight and thin thickness. 相似文献
67.
气相色谱离子阱质谱联用仪(GC-ITMS)广泛地应用于药物分析、环境分析、农药检测和食品分析、有机化学品分析、毒品分析以及医学和生物分析等领域。离子阱质谱作为色谱的检测器,决定了色质联用仪的分析性能,包括检出限、分辨率。离子阱质量分析器从传统的双曲型3D离子阱发展到2D线性离子阱,质量歧视效应得到了极大的改善,灵敏度得到了提高。矩形离子阱作为线性离子阱,结构简单,加工和装配容易,因此应用到GCMS系统中将具有非常大的优势。介绍了矩形离子阱质谱仪的设计方案、仪器整机的性能测试、质量分辨和质量歧视效应分析,与Agilent6890组成GCMS联用仪,对实际样品进行了分析。 相似文献
68.
69.
Jinlong Cai Yongtong Xiong Xiang Zhu Jinyu Hu Yunping Wang Junkai Li Jianfeng Wu Qinglai Wu 《Molecules (Basel, Switzerland)》2022,27(15)
The Kleier model and Carrier-mediated theory are effective for molecularly designing pesticides with phloem mobility. However, the single Kleier model or Carrier-mediated theory cannot achieve a reliable explanation of the phloem mobility of all exogenous substances. A detailed investigation of the two models and the scope of their applications can provide a more accurate and highly efficient basis for the guidance of the design and development of phloem-mobile pesticides. In the present paper, a strategy using active ingredient-amino acid conjugates as mode compounds is developed based on Carrier-mediated theory. An N-alkylated amino acid is used to improve the pesticide’s physicochemical properties following the Kleier model, thus allowing the conjugates to fall on the predicted and more accessible transportation region of phloem. Moreover, the influence of this movement on phloem is inspected by the Kleier model and Carrier-mediated theory. To verify this strategy, a series of N-alkylated phenazine-1-carboxylic acid-glycine compounds (PCA-Gly) were designed and synthesized. The results related to the castor bean seeds (R. communis L.) indicated that all the target compounds (4a–4f) had phloem mobility. The capacity for phloem mobility shows that N-alkylated glycine containing small substituents can significantly improve PCA phloem mobility, such as 4c(i-C3H7-N) > 4a(CH3-N) ≈ 4b(C2H5-N) > 4d (t-C4H9-N) > PCA-Gly > 4e(C6H5-N) > 4f(CH2COOH-N), with an oil–water partition coefficient between 1.2~2.5. In particular, compounds 4a(CH3-N), 4b(C2H5-N), and 4c(i-C3H7-N) present better phloem mobility, with the average concentrations in phloem sap of 14.62 μΜ, 13.98 μΜ, and 17.63 μΜ in the first 5 h, which are 8 to 10 times higher than PCA-Gly (1.71 μΜ). The results reveal that the Kleier model and Carrier-mediated theory play a guiding role in the design of phloem-mobile pesticides. However, the single Kleier model or Carrier-mediated theory are not entirely accurate. Still, there is a synergism between Carrier-mediated theory and the Kleier model for promoting the phloem transport of exogenous compounds. Therefore, we suggest the introduction of endogenous plant compounds as a promoiety to improve the phloem mobility of pesticides through Carrier-mediated theory. It is necessary to consider the improvement of physicochemical properties according to the Kleier model, which can contribute to a scientific theory for developing phloem-mobile pesticides. 相似文献
70.
Liangcong Hu Xudong Xie Hang Xue Tiantian Wang Adriana C. Panayi Ze Lin Yuan Xiong Faqi Cao Chengcheng Yan Lang Chen Peng Cheng Kangkang Zha Yun Sun Guodong Liu Chenyan Yu Yiqiang Hu Ranyang Tao Wu Zhou Bobin Mi Guohui Liu 《Experimental & molecular medicine》2022,54(7):961
MicroRNAs (miRNAs) broadly regulate normal biological functions of bone and the progression of fracture healing and osteoporosis. Recently, it has been reported that miR-1224-5p in fracture plasma is a potential therapy for osteogenesis. To investigate the roles of miR-1224-5p and the Rap1 signaling pathway in fracture healing and osteoporosis development and progression, we used BMMs, BMSCs, and skull osteoblast precursor cells for in vitro osteogenesis and osteoclastogenesis studies. Osteoblastogenesis and osteoclastogenesis were detected by ALP, ARS, and TRAP staining and bone slice resorption pit assays. The miR-1224-5p target gene was assessed by siRNA-mediated target gene knockdown and luciferase reporter assays. To explore the Rap1 pathway, we performed high-throughput sequencing, western blotting, RT-PCR, chromatin immunoprecipitation assays and immunohistochemical staining. In vivo, bone healing was judged by the cortical femoral defect, cranial bone defect and femoral fracture models. Progression of osteoporosis was evaluated by an ovariectomy model and an aged osteoporosis model. We discovered that the expression of miR-1224-5p was positively correlated with fracture healing progression. Moreover, in vitro, overexpression of miR-1224-5p slowed Rankl-induced osteoclast differentiation and promoted osteoblast differentiation via the Rap1-signaling pathway by targeting ADCY2. In addition, in vivo overexpression of miR-1224-5p significantly promoted fracture healing and ameliorated the progression of osteoporosis caused by estrogen deficiency or aging. Furthermore, knockdown of miRNA-1224-5p inhibited bone regeneration in mice and accelerated the progression of osteoporosis in elderly mice. Taken together, these results identify miR-1224-5p as a key bone osteogenic regulator, which may be a potential therapeutic target for osteoporosis and fracture nonunion.Subject terms: Translational research, Cell signalling 相似文献