全文获取类型
收费全文 | 1553篇 |
免费 | 167篇 |
国内免费 | 178篇 |
专业分类
化学 | 757篇 |
晶体学 | 14篇 |
力学 | 46篇 |
综合类 | 5篇 |
数学 | 113篇 |
物理学 | 346篇 |
综合类 | 617篇 |
出版年
2024年 | 4篇 |
2023年 | 31篇 |
2022年 | 48篇 |
2021年 | 51篇 |
2020年 | 59篇 |
2019年 | 55篇 |
2018年 | 46篇 |
2017年 | 59篇 |
2016年 | 70篇 |
2015年 | 72篇 |
2014年 | 83篇 |
2013年 | 110篇 |
2012年 | 139篇 |
2011年 | 104篇 |
2010年 | 88篇 |
2009年 | 78篇 |
2008年 | 109篇 |
2007年 | 77篇 |
2006年 | 95篇 |
2005年 | 55篇 |
2004年 | 44篇 |
2003年 | 33篇 |
2002年 | 30篇 |
2001年 | 25篇 |
2000年 | 33篇 |
1999年 | 43篇 |
1998年 | 36篇 |
1997年 | 30篇 |
1996年 | 30篇 |
1995年 | 19篇 |
1994年 | 21篇 |
1993年 | 18篇 |
1992年 | 20篇 |
1991年 | 18篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1964年 | 1篇 |
1937年 | 1篇 |
排序方式: 共有1898条查询结果,搜索用时 15 毫秒
971.
预应力扩大头锚索作为锚杆支护的新工艺、新技术,近年来较为广泛地应用于深基坑支护工程中,其施工技术也日趋成熟.相比普通锚杆,扩大头锚索具有单根抗拔力大、位移小、可靠性高、经济性好等特点,其支护效果亦得到业内认可. 相似文献
972.
研究了基于依赖度的属性约简算法,并以实例说明该算法不适合于不相容的决策表,在分析算法产生缺陷原因的同时通过改变算法求核的终止条件,使得算法能够同时适用于相容和不相容的决策表.在求约简过程当中,首先对除核之外剩余属性重要度进行计算,然后以重要度大小依次添加入核求约简.最后通过实例来验证改进后算法的有效性. 相似文献
973.
可修复产品逆向物流网络的鲁棒优化模型研究 总被引:1,自引:0,他引:1
考虑一类同时具有回收、再处理、处理和废物处理中心的可修复产品逆向物流网络在产品回收率不确定环境下的网络优化问题.为了提高可修复产品逆向物流网络的经济效益和网络的稳健性,建立了可修复产品逆向物流网络在产品回收率不确定情况下的鲁棒优化模型,分析了鲁棒优化模型解的存在性. 相似文献
974.
以橘子皮(OP)为原料通过MgCl2改性制备新型橘子皮吸附剂MgOP.考察溶液pH、固液比、温度、吸附时间和金属离子质量浓度对其从水溶液中吸附Cd2+和Ni2+的吸附性能的影响.采用扫描电镜及红外光谱仪对吸附剂进行表征.MgOP对2种金属离子的吸附率随pH和固液比的增大而增大;温度对吸附率的影响较小;吸附速度很快,能在20 min内达到吸附平衡.MgOP对Cd2+和Ni2+的吸附动力学均符合准二级动力学方程;MgOP对Cd2+和Ni2+的的Langmuir最大吸附量分别为125.47和44.58mg/g. 相似文献
975.
In order to study the probability for heavy ions to have a long projectile range in botanic sample, transmission energy spectra of 1.5 MeV F+, 3 MeV F2+ and 1.5 MeV H+ penetrating 100 µm seed cotyledon samples were measured as a function of ion dose. Results show that very fewer ions can penetrate through the samples, though their theoretical ranges are much shorter than sample thickness. Besides, the measured energy spectra of 1.5 MeV and 3.0 MeV F ions change dynamically while increasing the ion dose, they extend to the high energy direction and the count rates of the transmission ions increases quickly. These phenomena can be understood with the special composition and structure of the biological material. 相似文献
976.
The yields of fragments produced in the 60Ni+ 12C reactions at 80 A and 140 A MeV, and with maximum impact parameters of 1.5, 2 and 7.3 fm at 80 A MeV are calculated by the statistical abrasion-ablation model. The yields of fragments are analyzed by the isobaric yield ratio (IYR) method to extract the coefficient of symmetry energy to temperature (asym/T ). The incident energy is found to influence asym/T very little. It’s found that asym/T of fragments with the same neutron-excess I = N-Z increases when A increases, while asym/T of isobars decreases when A increases. The asym/T of prefragments is rather smaller than that of the final fragments, and the asym/T of fragments in small impact parameters is smaller than that of the larger impact parameters, which both indicate that asym/T decreases when the temperature increases. The choice of the reference IYRs is found to have influence on the extracted a sym /T of fragments, especially on the results of the more neutron-rich fragments. The surface-symmetry energy coefficient (bs/T ) and the volume-symmetry energy coefficient (bv/T) are also extracted, and the bs/bv is found to coincide with the theoretical results. 相似文献
977.
Yanchao Sha Yuanxiang Zhou Ling Zhang Meng Huang Fubao Jin 《Journal of Electrostatics》2013,71(3):540-546
Depending on unique operation characteristics for voltage applied on valve side winding of the converter transformers, it is extraordinarily significant to study the partial discharge (PD) behavior with oil-paper insulation under combined AC–DC voltage. Therefore, this paper investigated PD inception characteristics by pulse current methods with needle-plate electrode system under combined AC–DC voltage. Furthermore, 3D electric field distributions versus combined AC–DC voltage in different ratios were calculated by Finite Element Analysis (FEA). An experimental conclusion was drew that AC partial discharge inception voltage (PDIV) in pure oil would decrease linearly with the DC component increasing but the inception voltage with oil-paper insulation appeared to be independent of DC voltage and dependent of AC voltage. And 3D electric field distribution deduced from simulation provided a supplementary proof on the experimental results. Moreover, high speed photography was used to capture emitted light produced by discharge, estimate streamer velocity (1.8 km/s) and record streamer initiation and propagation process in oil gap. Previous studies have shown that the prebreakdown phenomena involving the generation and propagation of vapor channels through the oil could be divided into a three-stage process. 相似文献
978.
Abstract Effect of high pressure on the crystal structure of rhombohedral NaNO3 was investigated by X-ray diffraction of single crystals mounted in a miniature diamond-anvil cell on synchrotron radiation source. Diffraction intensity measurements were made at three pressures across a suggested transition pressure 4.3 GPa. No change was observed in an overall distribution of reflections in the reciprocal space with increasing pressure, but there was a systematic variation in diffraction intensity for particular groups of reflections. An analysis based on the structure factor calculation showed that a structure change induced by pressure is mainly a rotation of the nitrate groups in the alternate layers along the threefold axis in opposite directions. Least-squares refinement of the atomic positional parameters yielded the angle of the rotation to be 4.3 and 7.0 deg at pressures of 4.4 and 5.0 GPa, respectively. It has also been shown that the positions of the sodium and nitrogen atoms are slightly displaced along the axis, resulting in the formation of dipoles in the high pressure phase. 相似文献
979.
980.
为计算有耗色散光子晶体的带隙结构,提出了新的本征值分析方法.该方法借助于量子输运问题中的思想,在本征值方程的推导过程中进行了巧妙的变换,将复杂的非线性本征值问题转化为线性本征值问题:并利用频域有限差分(FDFD)方法直接求解线性本征值方程,最终得到有耗色散光子晶体结构的相关物理参数.与其他方法相比,该方法的最大特点为概念清晰、计算简便,最终节省了计算时间及所需内存量.利用该方法,对介质光子晶体结构进行模拟,结果与传统FDFD方法符合较好,从而验证了方法的有效性.此外,利用所提方法计算了有耗色散光子晶体结构的色散曲线,得到了表面等离子波激发的区域,进一步讨论了损耗对其色散曲线及本征模场的影响.相关结果对色散有耗光子晶体的研究具有一定的理论指导意义. 相似文献