全文获取类型
收费全文 | 1653篇 |
免费 | 252篇 |
国内免费 | 155篇 |
专业分类
化学 | 891篇 |
晶体学 | 21篇 |
力学 | 258篇 |
综合类 | 37篇 |
数学 | 79篇 |
物理学 | 774篇 |
出版年
2024年 | 5篇 |
2023年 | 23篇 |
2022年 | 43篇 |
2021年 | 67篇 |
2020年 | 68篇 |
2019年 | 49篇 |
2018年 | 37篇 |
2017年 | 69篇 |
2016年 | 74篇 |
2015年 | 68篇 |
2014年 | 57篇 |
2013年 | 182篇 |
2012年 | 103篇 |
2011年 | 89篇 |
2010年 | 83篇 |
2009年 | 84篇 |
2008年 | 67篇 |
2007年 | 100篇 |
2006年 | 96篇 |
2005年 | 67篇 |
2004年 | 55篇 |
2003年 | 64篇 |
2002年 | 55篇 |
2001年 | 61篇 |
2000年 | 49篇 |
1999年 | 40篇 |
1998年 | 50篇 |
1997年 | 35篇 |
1996年 | 29篇 |
1995年 | 37篇 |
1994年 | 38篇 |
1993年 | 24篇 |
1992年 | 13篇 |
1991年 | 12篇 |
1990年 | 18篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1959年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有2060条查询结果,搜索用时 234 毫秒
121.
J. Pablo Lamas Lucia Sanchez-PradoMarta Lores Carmen Garcia-JaresMaria Llompart 《Journal of chromatography. A》2010,1217(33):5307-5316
Exposure to fragrance substances is exponentially increasing in our daily life due to the enhanced use of scented products. Some fragrances are known to be important sensitizers, inhalation being an important exposure pathway in indoor environments. A simple and sensitive method based on solid-phase enrichment and solid-phase microextraction (SPME) followed by gas chromatography–mass spectrometry (GC–MS) has been developed for the analysis of 24 volatile fragrance allergens in indoor air. Suspected allergens present in the air (0.2 m3) were adsorbed onto a very small quantity of florisil (25 mg) and then transferred to a SPME fiber in the headspace mode (HS). To the best of our knowledge, this paper describes the first application of SPME for the determination of these compounds in air samples. The experimental parameters affecting the microextraction process have been optimized using a multifactor experimental design strategy. Accuracy, linearity, precision and detection limits (LODs) were evaluated to assess the performance of the proposed method. External calibration, using spiked sorbent standards, and not requiring the complete sampling process (only the SPME step), demonstrated to be suitable for the quantification of all suspected allergens. Recovery studies were performed at three concentration levels (0.04, 1.00 and 50 μg m−3), obtaining quantitative recoveries (≥85%) in most cases. LOD values at the low ng m−3 level were achieved for all the target compounds. The application of the method to daily home air samples demonstrated the ubiquity of this kind of fragrance ingredients in quotidian indoor environments, finding 18 of the 24 considered compounds in concentrations ranging from 0.01 to 56 μg m−3. Benzyl alcohol, linalool, citronellol, ionone and lilial were found in most analyzed samples. 相似文献
122.
Lelio Zoccolillo Luca Amendola Susanna Insogna Elisabetta Pastorini 《Journal of chromatography. A》2010,1217(24):3890-3895
An analytical system composed of a cryofocusing trap injector device coupled to a gas chromatograph with mass spectrometric detection (CTI-GC–MS) specific for the on-line analysis in air of volatile chlorinated hydrocarbons (VCHCs) (dichloromethane; chloroform; 1,1,1-trichloroethane; tetrachloromethane; 1,1,2-trichloroethylene; tetrachloroethylene) was developed. The cryofocusing trap injector was the result of appropriate low cost modifications to an original purge-and-trap device to make it suitable for direct air analysis even in the case of only slightly contaminated air samples, such as those from remote zones. The CTI device can rapidly and easily be rearranged into the purge-and-trap allowing water and air analysis with the same apparatus. Air samples, collected in stainless steel canisters, were introduced directly into the CTI-GC–MS system to realize cryo-concentration (at −120 °C), thermal desorption (at 200 °C) and for the subsequent analysis of volatiles. The operating phases and conditions were customised and optimized. Recovery efficiency was optimized in terms of moisture removal, cold trap temperature and sampling mass flow. The injection of entrapped volatiles was realized through a direct transfer with high chromatographic reliability (capillary column–capillary column). These improvements allowed obtaining limits of detection (LODs) at least one order of magnitude lower than current LODs for the investigated substances. The method was successfully employed on real samples: air from urban and rural areas and air from remote zones such as Antarctica. 相似文献
123.
124.
125.
根据鱼雷对舰船毁伤的特点,建立了多介质水下爆炸数值模型,提出了交界面处理及广义光滑长度计算方法,编制了多介质水下爆炸计算程序,并对水下爆炸空气隔层衰减冲击波的性能进行了定量分析。结果表明:无论是接触爆炸还是非接触爆炸,空气隔层均可有效衰减冲击波,并且接触爆炸中空气隔层衰减冲击波的效果更好,最大可使冲击压力峰值降低约55%;空气隔层厚度与爆炸厚度之比为1时便可达到较好的衰减冲击波效果,继续增加空气隔层厚度对衰减冲击波效果影响不大。研究结果可为舰船结构防护及防雷舱的结构设计提供参考。 相似文献
126.
Rotating-detonation-engines (RDE’s) represent an alternative to the extensively studied pulse-detonation-engines (PDE’s) for obtaining propulsion from the high efficiency detonation cycle. Since it has received considerably less attention, the general flow-field and effect of parameters such as stagnation conditions and back pressure on performance are less well understood than for PDE’s. In this article we describe results from time-accurate calculations of RDE’s using algorithms that have successfully been used for PDE simulations previously. Results are obtained for stoichiometric hydrogen–air RDE’s operating at a range of stagnation pressures and back pressures. Conditions within the chamber are described as well as inlet and outlet conditions and integrated quantities such as total mass flow, force, and specific impulse. Further computations examine the role of inlet stagnation pressure and back pressure on detonation characteristics and engine performance. The pressure ratio is varied between 2.5 and 20 by varying both stagnation and back pressure to isolate controlling factors for the detonation and performance characteristics. It is found that the detonation wave height and mass flow rate are determined primarily by the stagnation pressure, whereas overall performance is closely tied to pressure ratio. Specific impulses are calculated for all cases and range from 2872 to 5511 s, and are lowest for pressure ratios below 4. The reason for performance loss is shown to be associated with the secondary shock wave structure that sets up in the expansion portion of the RDE, which strongly effects the flow at low pressure ratios. Expansion to supersonic flow behind the detonation front in RDE’s with higher pressure ratios isolate the detonation section of the RDE and thus limit the effect of back pressure on the detonation characteristics. 相似文献
127.
研制了一套具有4条测量光路的主动差分光学吸收光谱仪(DOAS)系统,以一盏短孤氙灯为4台集成一体的发射/接收望远镜提供光源,可同时发射4条相互独立的测量光路,4台望远镜的角度可分别独立调整以对准不同位置的角反射镜,光路之间的最大发散角可达28°.为测试仪器性能,使4条光路测量同一区域内的NO2浓度. 对4条光路的测量结果进行相互对比,任意两条光路的相关系数均在0.97以上,测得NO2的体积分数平均差异小于10-9, 表明结果具有良好的一致性
关键词:
差分光学吸收光谱
2')" href="#">NO2
2')" href="#">SO2
大气污染 相似文献
128.
129.
为了均衡终端区扇区的工作负荷,保障扇区边界的合理性,研究了终端扇区的划分方法.针对建立的扇区划分数学模型,提出了3阶段的划分方法.首先,采用voronoi图和遗传算法完成初始划分,实现均衡负荷的目标;其次,提出了基于voronoi图和基于maklink图的2种扇区二次划分方法,满足最小飞行时间及最小距离约束;最后,应用分段线性拟合的方法消除扇区边界的锯齿状,以及满足扇区凸形约束.以广州终端为例进行了仿真计算.应用提出的方法,给出了2种划分结果,都能够使得各扇区的工作负荷在2880s以内,差值不超过350s,扇区边界较为平滑,且计算时间明显减少.仿真计算结果说明方法是可行和有效的,也为使用者提供了灵活的选择. 相似文献
130.
Yongzheng Wang Jing LiJiyang Wang Shujuan HanYongjie Guo 《Journal of Crystal Growth》2010,312(19):2779-2782
Neodymium phosphate single crystals, NdPO4, have been grown by a flux growth method using Li2CO3-2MoO3 as a flux. The as-grown crystals were characterized by X-ray powder diffraction(XRPD), differential thermal analysis (DTA) and thermogravimetric analysis (TG) techniques. The results show that the as-grown crystals were well crystallized. The crystal was stable over the temperature range from 26 to 1200 °C in N2. The specific heat of NdPO4 crystal at room temperature was 0.41 J/g °C. The absorption and the fluorescence spectra of NdPO4 crystal were also measured at room temperature. 相似文献