全文获取类型
收费全文 | 3237篇 |
免费 | 553篇 |
国内免费 | 336篇 |
专业分类
化学 | 2331篇 |
晶体学 | 37篇 |
力学 | 197篇 |
综合类 | 18篇 |
数学 | 341篇 |
物理学 | 1202篇 |
出版年
2024年 | 13篇 |
2023年 | 104篇 |
2022年 | 122篇 |
2021年 | 156篇 |
2020年 | 173篇 |
2019年 | 148篇 |
2018年 | 143篇 |
2017年 | 98篇 |
2016年 | 190篇 |
2015年 | 164篇 |
2014年 | 201篇 |
2013年 | 214篇 |
2012年 | 307篇 |
2011年 | 333篇 |
2010年 | 196篇 |
2009年 | 228篇 |
2008年 | 211篇 |
2007年 | 165篇 |
2006年 | 153篇 |
2005年 | 120篇 |
2004年 | 95篇 |
2003年 | 76篇 |
2002年 | 72篇 |
2001年 | 58篇 |
2000年 | 51篇 |
1999年 | 56篇 |
1998年 | 43篇 |
1997年 | 35篇 |
1996年 | 52篇 |
1995年 | 24篇 |
1994年 | 30篇 |
1993年 | 15篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1974年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有4126条查询结果,搜索用时 15 毫秒
151.
In this paper, we establish global well-posedness and asymptotic stability of mild solutions for the Cauchy problem of the fractional drift-diffusion system with small initial data in critical Besov spaces. The regularizing-decay rate estimates of mild solutions are also proved, which imply that mild solutions are analytic in space variables. 相似文献
152.
旅游地摄影是旅游者自身对目的地的视觉表征,通过潜在游客对目的地的形象感知影响其旅游决策.旅游地摄影被旅游企业视为旅游地视觉营销的重要策略.基于跨文化视角,以中美旅游者拍摄且上传网络的西藏旅游照片为研究对象,采用内容分析法、卡方检验、社会网络分析法和空间分析法,借助NVivo11.0、SPSS22.0、Ucinet6和ArcGIS10.2软件,探寻中美游客摄影图片对西藏视觉表征的差异,以期为旅游目的地定向营销提供参考依据.研究发现:(1)从照片编码树状节点频数来看,雄壮瑰丽的自然景观、虔诚的佛教信徒、神秘的民族非物质文化是西藏旅游形象的核心要素和符号.(2)从照片树状节点类目间差异分析来看,两组照片在自然景观、民族非物质文化、户外活动、舞台化表演和衣饰6个类目上存在统计学差异.中国旅游者的照片更倾向于表征自然景观、当地人生活细节、户外活动和精心设计的舞台化表演.美国游客的照片则更侧重于对原真性的民族非物质文化和绚丽多彩、独特别致的民族服饰的表征.(3)从照片"聚合形象地图"来看,美国样本的聚合形象地图较为复杂和多样化,而中国样本则相对简单有条理.(4)从中美游客地理摄影偏好来看,游客最爱拉萨.除此之外,中国游客更为偏爱那曲地区,而美国游客却更倾向于山南和日喀则地区. 相似文献
153.
In this paper the conservation theorems of the constrained Birkhoffian systems are
studied by using the method of integrating factors. The differential equations of
motion of the system are written. The definition of integrating factors is given
for the system. The necessary conditions for the existence of the conserved quantity
for the system are studied. The conservation theorem and its inverse for the system
are established. Finally, an example is given to illustrate the application of the
results. 相似文献
154.
Raman spectra of 5.0 and 8.4 M aqueous KSCN solutions were recorded. The spectra were analyzed by memory-function and stretched-exponential modeling procedures. The two procedures modeled the vibrational autocorrelation functions within experimental error. The success of the stretched exponential model was found to be its very good fit to the memory functions. 相似文献
155.
Junwen Lu Wei Wang Aiqing Zhang Fatang Tan Xueliang Qiao 《Journal of Sol-Gel Science and Technology》2012,63(3):495-500
Hybrid nanocomposite films of magnesium oxide (MgO) in fluorinated polyimide (PI) from 4, 4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 4, 4′-Diaminodiphenyl ether (ODA) have been successfully fabricated via an in situ sol–gel polymerization technique. The MgO content in hybrid films was varied from 0 to 5 wt%. The hybrid films were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), ultraviolet–visible (UV–Vis) spectroscopy and thermal gravimetric analyses (TGA). The results of FTIR, XRD and FESEM showed that the MgO nanoparticles were well dispersed in the polymer matrix due to the coordination between the carbonyl group of polymers and Mg atom, and the as-prepared hybrid films exhibited excellent optical transparency in the visible region and good UV-shielding properties in the UV region. Although the thermal stability of the hybrid films is slight inferior to pure PI, it is still good for the practical application below the temperature of 300 °C. 相似文献
156.
超导共面波导谐振腔主要包括四分之一波长型和半波长型.本文对半波长超导共面波导谐振腔使用并联RLC电路进行等效分析,使用sonnet软件进行仿真其传输特性,通过电子束蒸发和激光直写等工艺制备出样品,在低温超导状态下使用矢量网络分析仪对其传输特性进行测量,以及通过持续降温方式研究其传输特性受温度的影响.实验表明:测得的谐振频率和外部品质因数与设计值吻合;谐振频率受动态电感的影响随温度降低而升高;外部品质因数随温度降低而保持基本不变;内部品质因先受导体损耗的影响随温度降低而急剧升高,当温度降低至一定温度后内部品质因数受介电损耗的影响随温度降低而保持基本不变;总品质因数在温度较高时主要来自于导体损耗,在温度较低时主要来自于介电损耗和外部品质因数,随温度的变化趋势和内部品质因数一致. 相似文献
157.
158.
Qiao Liangzhi Wang Shanshan Wang Tao Yu Shasha Guo Shaohui Du Kaifeng 《Cellulose (London, England)》2021,28(14):9323-9333
Cellulose - Dye contamination of water supplies has a serious threat to human health, prompting the development of highly effective and eco-friendly adsorbents. In this work, polyelectrolyte... 相似文献
159.
We have developed an efficient method for the synthesis of primary arylamines from aryl halides using amidine hydrochlorides as the ammonia surrogates. The protocol uses 10 mol % CuI as the catalyst, 20 mol % L-proline as the ligand, Cs2CO3 as the base, and DMF as the solvent and proceeds the sequential coupling of aryl halides with amidine hydrochlorides and hydrolysis of intermediates to give the target products. This is a convenient, inexpensive, and practical approach to primary arylamines. 相似文献
160.
The one-carbon unit transfer reaction catalysed by glycinamide ribonucleotide transformylase(GARTfase)is a key step in the de novo purine biosynthetic pathway. In order to give a theoretical research of the assumption from the experiments,the water-assisted mechanism in GAR Tfase catalysed one-carbon unit transfer reaction has been investigated by a Density Functional Theory method,B3LYP,at 6-31G* basis level. There are two possible reaction channels for the whole reaction,one is concerted(path a)and the other is stepwise(path b). The water molecule serve as a bridge to connect the proton donor to the proton acceptor. All the transition states in both paths have a six-membered ring in their structures due to the joint of the water molecule. The calculations show that the latter is preferable to the former due to the lower energy barriers. The results have verified the presumption from experiments,and proved that the joint of a water molecule can relax the strong strain in the unstable system,so it is propitious to the whole reaction. 相似文献