全文获取类型
收费全文 | 13201篇 |
免费 | 1505篇 |
国内免费 | 1516篇 |
专业分类
化学 | 6127篇 |
晶体学 | 94篇 |
力学 | 591篇 |
综合类 | 104篇 |
数学 | 996篇 |
物理学 | 2813篇 |
综合类 | 5497篇 |
出版年
2024年 | 72篇 |
2023年 | 234篇 |
2022年 | 411篇 |
2021年 | 456篇 |
2020年 | 432篇 |
2019年 | 379篇 |
2018年 | 349篇 |
2017年 | 344篇 |
2016年 | 490篇 |
2015年 | 486篇 |
2014年 | 688篇 |
2013年 | 848篇 |
2012年 | 940篇 |
2011年 | 993篇 |
2010年 | 730篇 |
2009年 | 762篇 |
2008年 | 885篇 |
2007年 | 808篇 |
2006年 | 662篇 |
2005年 | 659篇 |
2004年 | 503篇 |
2003年 | 387篇 |
2002年 | 429篇 |
2001年 | 361篇 |
2000年 | 316篇 |
1999年 | 335篇 |
1998年 | 318篇 |
1997年 | 303篇 |
1996年 | 273篇 |
1995年 | 249篇 |
1994年 | 217篇 |
1993年 | 169篇 |
1992年 | 159篇 |
1991年 | 138篇 |
1990年 | 105篇 |
1989年 | 99篇 |
1988年 | 83篇 |
1987年 | 58篇 |
1986年 | 31篇 |
1985年 | 26篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1974年 | 1篇 |
1959年 | 1篇 |
1957年 | 3篇 |
1955年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Miao Cheng Zongyi Qin Yuanyu Chen Jiaming Liu Zichu Ren 《Cellulose (London, England)》2017,24(8):3243-3254
A facile and efficient approach to prepare carboxylated cellulose nanocrystals (CCNCs) is presented through a novel one-step hydrothermal procedure by using a mixed acid system of hydrochloric acid and nitric acid (HCl/HNO3). The as-prepared cellulose nanoparticles were characterized by scanning electron microscopy, wide angle X-ray diffraction, conductometric titrations, Fourier transform infrared spectrometry and thermal gravimetric analysis. The results showed that the combination of the mixed acid and hydrothermal reaction can speed up the process of CCNC preparation, and then high quality of the product could be obtained at relatively low acid concentration. It is found that the addition of nitric acid could not only promote the conversion of surface groups on the cellulose nanocrystals (CNCs), but also have significant influences on the yield, particle size and microstructure of CNCs. For the volume ratio of HCl/HNO3 of 7:3, the as-prepared CCNCs exhibited the largest length to diameter ratio and narrowest dimension distributions as well as maximum degree of oxidation of 0.12. In addition, high dispersion stability for the CCNCs could be observed due to the existence of negative carboxyl groups. This approach based on one-step oxidative carboxylation greatly simplified the preparation of CCNCs with high yield and high crystallinity under mild hydrothermal condition. 相似文献
992.
Yaru Qin Qian Gao Yanmei Chen Wei Liu Feng Lin XiangFei Zhang Yaping Dong Yahong Li 《Journal of Cluster Science》2017,28(3):891-903
The incorporation of LnIII ions into the 12-metallacrown-4 topology affords the formation of four mixed 3d-4f pentanuclear complexes of compositions [NH(C2H5)3]{[Ln(OAc)4] [12-MC Mn III (N)shi-4]}·xH2O (Ln = Sm (1), Gd (2), Tb (3), Dy (4); x = 0.5 for 1 and 3, x = 0.25 for 2, x = 0 for 4; H3shi = salicylhydroxamic acid). Compounds 1–4 were obtained from the reactions of H3shi with Mn(CH3COO)2·4H2O and Ln(NO3)3·6H2O, in the presence of N(C2H5)3. They all contain a crown-like [Mn4Ln(μ-NO)4]11+ core with four MnIII atoms being at the rim of the crown and an LnIII ion occupying the dome of the crown. The peripheral ligation about the core is provided by four η1:η1:µ acetate groups. The identity of the LnIII ions slightly affects the 12-metallacrown-4 frameworks, as demonstrated by the gradual decrease of the distances between the LnIII ions and the centres of the Mn4 planes (1.85 Å for 1, 1.81 Å for 2, 1.80 Å for 3, and 1.77 Å for 4). Variable-temperature dc magnetic susceptibility studies were carried out on polycrystalline samples of 1–4. Antiferromagnetic interactions are determined for complexes 1–4. 相似文献
993.
994.
995.
联营型B2C平台通过自建物流改善服务质量,却面临建设中巨额成本问题,而平台上的联营企业虽无较大成本压力,但其服务质量却有待改进。基于联营企业与其所在平台的佣金利益关系,从双方销售同种产品直接竞争的角度出发,运用合作竞争理论的思想构建模型并进行分析。结果表明,在满足一定条件的基础上,帕累托改进是存在的,自营与联营企业可以相互合作来实现双赢,且商家能通过控制各因素而实现对价格和销量的不同偏好。 相似文献
996.
997.
熊琴 《纯粹数学与应用数学》2017,33(3)
研究了线性模型中广义最小二乘参数估计的误差分布稳健性问题.首先讨论了在线性统计模型里,设计矩阵为列降秩矩阵时,模型中给出了误差最大分布类,当误差向量的分布在此范围内变动时,LS估计和GLS估计在均方误差矩阵准则下是最优估计.然后进一步探讨广义最小二乘估计GLSE关于误差分布的稳健性,求出误差项所对应的最大分布族,进而证明了在该区间波动情况下,误差向量对应的始终为一致最优解. 相似文献
998.
999.
Meshless method with ridge basis functions 总被引:1,自引:0,他引:1
Meshless collocation methods for the numerical solutions of PDEs are increasingly adopted due to their advantages including efficiency and flexibility, and radial basis functions are popularly employed to represent the solutions of PDEs. Motivated by the advantages of ridge basis function representation of a given function, such as the connection to neural network, fast convergence as the number of terms is increased, better approximation effects and various applications in engineering problems, a meshless method is developed based on the collocation method and ridge basis function interpolation. This method is a truly meshless technique without mesh discretization: it neither needs the computation of integrals, nor requires a partition of the region and its boundary. Moreover, the method is applied to elliptic equations to examine its appropriateness, numerical results are compared to that obtained from other (meshless) methods, and influence factors of accuracy for numerical solutions are analyzed. 相似文献
1000.
In this paper, we initiate the solvability and stability for a class of singular fractional $(p,q)$-difference equations.
First, we obtain an existence theorem of solution for the fractional $(p,q)$-difference equation. Then, by using a fractional $(p,q)$-Gronwall inequality, some stability criteria of solution are established, which also implies the uniqueness of solution. 相似文献