全文获取类型
收费全文 | 4193篇 |
免费 | 401篇 |
国内免费 | 351篇 |
专业分类
化学 | 968篇 |
晶体学 | 46篇 |
力学 | 376篇 |
综合类 | 100篇 |
数学 | 1911篇 |
物理学 | 1544篇 |
出版年
2024年 | 6篇 |
2023年 | 55篇 |
2022年 | 87篇 |
2021年 | 98篇 |
2020年 | 105篇 |
2019年 | 106篇 |
2018年 | 92篇 |
2017年 | 116篇 |
2016年 | 135篇 |
2015年 | 115篇 |
2014年 | 160篇 |
2013年 | 276篇 |
2012年 | 179篇 |
2011年 | 184篇 |
2010年 | 215篇 |
2009年 | 265篇 |
2008年 | 257篇 |
2007年 | 288篇 |
2006年 | 233篇 |
2005年 | 222篇 |
2004年 | 243篇 |
2003年 | 195篇 |
2002年 | 209篇 |
2001年 | 169篇 |
2000年 | 169篇 |
1999年 | 121篇 |
1998年 | 129篇 |
1997年 | 100篇 |
1996年 | 78篇 |
1995年 | 66篇 |
1994年 | 66篇 |
1993年 | 28篇 |
1992年 | 34篇 |
1991年 | 23篇 |
1990年 | 26篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有4945条查询结果,搜索用时 46 毫秒
141.
The results of mineralogical studies of technogenic tungsten raw material (stale tailings of Bom-Gorhon deposit) are represented. Its particle size distribution as well as tungsten and accompanying element distribution among the fractions were determined. The necessity of grinding the heaps down to 0.2-0.25 mm in size was established. It allows increasing the recovery rate of two or more times in comparison with the traditional pattern of tailing processing. 相似文献
142.
为建设适应新时代要求,突出高阶性、创新性、挑战度的一流本科课程,实现价值引领、能力培养和知识传授的教育教学目标,结合功能材料专业的特点和民族高等院校学生的学情,构建了物理化学基础课程的“四模块、三阶段、两时空、双平台”的混合式教学新模式,重塑了“价值引领模块、基础理论模块、多学科融合模块和虚拟仿真实验模块”的“四模块”内容体系。通过“四模块”嵌入下的混合式教学实践研究,结合学生的课前、课中、课后等学习成绩,多维度评价学习效果,并藉由调查问卷来分析学生对混合式教学的评价,最终研究结果表明学生的个性化自主学习能力有了显著提高、学生运用物理化学理论解决复杂问题的综合能力得到了提升,学生经过刻苦学习获得了素质提高的成就感。 相似文献
143.
《Arabian Journal of Chemistry》2022,15(4):103740
Lamiophlomis rotata (Benth.) Kudo (L. rotata) belongs to Lamiaceae family, which is an important medicinal plant endemic to Qinghai Tibet Plateau. Traditionally, the whole herb of L. rotata is used for medicine, especially for the treatment of rheumatoid arthritis (RA) in clinical practise. As a result of absolute digging, the plant has a long regeneration cycle after excavation and the damage to plateau grassland ecological environment is difficult to recover. It has been encouraged to use aerial part of the plant with the purpose of protecting environment and maintaning biological diversity. At present, researchers have compared the primary metabolites and iridoids between aerial parts and roots, but there are few reports on the chemical differences and activity comparison of secondary metabolites. In order to characterize the secondary metabolites of different parts, UPLC/Q-Orbitrap-MS was employed to collect data from the extracts of aerial parts and roots, in combination with plant metabolomics technology to screen and quantify differential metabolites. At the same time, network pharmacological analysis with rheumatoid arthritis and immunity as the key words was carried out according to the identification results to clarify the active ingredients of L. rotata in the treatment of RA, so as to speculate the pharmacological effects of aerial parts and roots based on the distribution of active components. A total of 16 potential markers were selected and identified to differentiate two parts. Among them, 8 characteristic flavonoids with similar skeletons were unique in aerial parts, while the other 8 components, including 2 iridoid glycosides and 6 phenylethanoid glycosides, were detected in both aerial parts and roots, but with differentiate contents. Among the predicted 6 active components, there were 5 flavonoids, of which 3 (namely luteolin, apigenin and 2″-acetylastragalin) were still differential metabolites and mainly distributed in the aerial parts. The results revealed that certain flavonoids as potential markers made a distinction between aerial part and root of L. rotata, and were the main active components against RA, which provided a theoretical basis for the aerial parts to replace the whole herbs, and laid a material foundation for further pharmacological research. 相似文献
144.
A drug nanocrystals self-stabilized Pickering emulsion (NSSPE) with a unique composition and microstructure has been proven to significantly increase the bioavailability of poorly soluble drugs. This study aimed to develop a new solid NSSPE of puerarin preserving the original microstructure of NSSPE by spray-drying. A series of water-soluble solid carriers were compared and then Box-Behnken design was used to optimize the parameters of spray-drying. The drug release and stability of the optimized solid NSSPE in vitro were also investigated. The results showed that hydroxypropyl-β-cyclodextrin (HP-β-CD), rather than solid carriers commonly used in solidification of traditional Pickering emulsions, was suitable for the solid NSSPE to retain the original appearance and size of emulsion droplets after reconstitution. The amount of HP-β-CD had more influences on the solid NSSPE than the feed rate and the inlet air temperature. Fluorescence microscopy, confocal laser scanning microscopy and scanning electron microscopy showed that the reconstituted emulsion of the solid NSSPE prepared with HP-β-CD had the same core-shell structure with a core of oil and a shell of puerarin nanocrystals as the liquid NSSPE. The particle size of puerarin nanocrystal sand interfacial adsorption rate also did not change significantly. The cumulative amount of released puerarin from the solid NSSPE had no significant difference compared with the liquid NSSPE, which were both significantly higher than that of puerarin crude material. The solid NSSPE was stable for 3 months under the accelerated condition of 75% relative humidity and 40 °C. Thus, it is possible todevelop the solid NSSPE preserving the unique microstructure and the superior properties in vitro of the liquid NSSPE for poorly soluble drugs. 相似文献
145.
A new and simple method, based entirely on a physical approach, was proposed to produce activated carbon from longan fruit seed with controlled mesoporosity. This method, referred to as the OTA, consisted of three consecutive steps of (1) air oxidation of initial microporous activated carbon of about 30% char burn-off to introduce oxygen surface functional groups, (2) the thermal destruction of the functional groups by heating the oxidized carbon in a nitrogen atmosphere at a high temperature to increase the surface reactivity due to increased surface defects by bond disruption, and (3) the final reactivation of the resulting carbon in carbon dioxide. The formation of mesopores was achieved through the enlargement of the original micropores after heat treatment via the CO2 gasification, and at the same time new micropores were also produced, resulting in a larger increase in the percentage of mesopore volume and the total specific surface area, in comparison with the production of activated carbon by the conventional two-step activation method using the same activation time and temperature. For the activation temperatures of 850 and 900 °C and the activation time of up to 240 min, it was found that the porous properties of activated carbon increased with the increase in activation time and temperature for both preparation methods. A maximum volume of mesopores of 0.474 cm3/g, which accounts for 44.1% of the total pore volume, and a maximum BET surface area of 1773 m2/g was achieved using three cycles of the OTA method at the activation temperature of 850 °C and 60 min activation time for each preparation cycle. The two-step activation method yielded activated carbon with a maximum mesopore volume of 0.270 cm3/g (33.0% of total pore volume) and surface area of 1499 m2/g when the activation temperature of 900 °C and a comparable activation time of 240 min were employed. Production of activated carbon by the OTA method is superior to the two-step activation method for better and more precise control of mesopore development. 相似文献
146.
147.
148.
多模光纤不同模式布里渊散射参数 总被引:1,自引:0,他引:1
基于射线光学和波动光学理论分析了多模光纤的布里渊散射特性,提出了确定布里渊散射角取值范围的方法;推导了阶跃型和渐变型多模光纤不同模式群布里渊频移、线宽、散射谱和散射功率的表达式.结果表明,阶跃型和渐变型多模光纤布里渊散射角的最大取值范围为全反射临界角的2倍到π;阶跃型光纤的布里渊频移、线宽、归一化峰值增益和散射功率随模式群的变化比渐变型光纤缓慢,且随着模式群编号的增加,阶跃型光纤的上述参量分别在11.084~10.932GHz、21.760~21.168 MHz、1~0.933和1.990×10-9~1.857×10-9 W范围内呈曲线下降;渐变型光纤的上述参量分别在11.064~10.969GHz、21.683~21.314MHz、1~0.957和2.052×10-9~1.965×10-9 W范围内呈直线下降. 相似文献
149.
In this paper, a novel structure for a dual-gated graphene nanoribbon field-effect transistor (GNRFET) is offered, which combines the advantages of high and low dielectric constants. In the proposed Two Different Insulators GNRFET (TDI-GNRFET), the gate dielectric at the drain side is a material with low dielectric constant to form smaller capacitances, while in the source side, there is a material with high dielectric constant to improve On-current and reduce the leakage current. Simulations are performed based on self-consistent solutions of the Poisson equation coupled with Non-Equilibrium Green's Function (NEGF) formalism in the ballistic regime. We assume a tight-binding Hamiltonian in the mode space representation. The results demonstrate that TDI-GNRFET has lower Off-current, higher On-current and higher transconductance in comparison with conventional low-K GNRFET. Furthermore, using a top-of-the-barrier two-dimensional circuit model, some important circuit parameters are studied. It is found that TDI-GNRFET has smaller capacitances, lower intrinsic delay time and shorter power delay product (PDP) in comparison with high-K GNRFET. Moreover, mobile charge and average velocity are improved in comparison with low dielectric constant GNRFET. The results show that the TDI-GNRFET can provide Drain Induced Barrier Lowering (DIBL) and Subthreshold Swing near their theoretical limits. 相似文献
150.
Xiaoxiao Wang Yixin Yao Chang An Xi Li Fangtao Xiang Yangli Dong Maosen Li 《Electrophoresis》2020,41(18-19):1606-1616
Chuanxiong Rhizoma is a commonly used in traditional Chinese medicine. Chuanxiong Rhizoma is widely distributed in Sichuan province, China, including the cities of Dujiangyan, Pengzhou, Meishan, Qionglai, and Shifang. However, reports on the comparisons of quality of Chuanxiong Rhizoma of different production origins are limited. Therefore, an ultra-HPLC with triple quadrupole MS method was developed for the determination of 20 bioactive components (12 aromatic acids and eight phthalides) in 36 samples from different production origins and further assessed its quality. The contents of these 20 constituents of samples were analyzed by hierarchical cluster analysis and orthogonal partial least squares discrimination analysis; the result indicated that Chuanxiong Rhizoma of different production origins had some differences. Thirteen constituents of quality difference markers were acquired by variable importance for the project. Furthermore, the sum of the contents of these quality difference markers was different from various production origins of Chuanxiong Rhizoma. Meanwhile, Z-ligustilide and senkyunolide A as main constituents of quality difference markers, the rate of various production origins of Chuanxiong Rhizoma was different. This study provides a foundation for the quality assessment of Chuanxiong Rhizoma. 相似文献