全文获取类型
收费全文 | 2126篇 |
免费 | 635篇 |
国内免费 | 286篇 |
专业分类
化学 | 1055篇 |
晶体学 | 22篇 |
力学 | 75篇 |
综合类 | 31篇 |
数学 | 276篇 |
物理学 | 1588篇 |
出版年
2024年 | 5篇 |
2023年 | 27篇 |
2022年 | 60篇 |
2021年 | 47篇 |
2020年 | 60篇 |
2019年 | 55篇 |
2018年 | 76篇 |
2017年 | 99篇 |
2016年 | 91篇 |
2015年 | 102篇 |
2014年 | 187篇 |
2013年 | 149篇 |
2012年 | 185篇 |
2011年 | 216篇 |
2010年 | 173篇 |
2009年 | 162篇 |
2008年 | 172篇 |
2007年 | 175篇 |
2006年 | 141篇 |
2005年 | 147篇 |
2004年 | 105篇 |
2003年 | 106篇 |
2002年 | 63篇 |
2001年 | 63篇 |
2000年 | 43篇 |
1999年 | 38篇 |
1998年 | 39篇 |
1997年 | 43篇 |
1996年 | 35篇 |
1995年 | 40篇 |
1994年 | 26篇 |
1993年 | 15篇 |
1992年 | 24篇 |
1991年 | 14篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 3篇 |
排序方式: 共有3047条查询结果,搜索用时 15 毫秒
41.
Mariana Santos Pinheiro Rita de Cássia Elias Estrela Marins Lúcio Mendes Cabral 《液相色谱法及相关技术杂志》2018,41(5):223-231
Roflumilast is a phosphodiesterase type 4 inhibitor that is administered orally as a long-term, in the clinical treatment of chronic obstructive pulmonary disease and asthma. Launched in 2010 for the European market, it currently does not have an official monograph. Here, a reproducible gradient RP-HPLC method was developed and validated for the separation and determination of Roflumilast in the presence of its six major degradation products. Separation was performed on a C18 analytical column (250?×?4.6?mm, 5?µm) with a mobile phase-A of ACN and a phase-B of ammonium acetate buffer (5?mM, pH 4.2) containing triethylamine (0.5% v/v). The most effective RP-HPLC gradient program was determined to be 0/80, 35/10, 36/80, 40/80 (time in minutes/% mobile phase-B). The flow rate was 1.0?ml/min and the column temperature was 25°C. The success of separation of the degradation products with different chemical characteristics was obtained by extending the time of the gradient, changing the proportion of the mobile phases and increasing the velocity of the flow. Two detectors were evaluated for the identification of degradation products and Roflumilast: a diode-arrary detector and a charged aerosol detector. The inability of the charged aerosol detector to dectect one of the six degradation products indicated that the method developed with RP-HPLC and the diode-array detector was more suitable for Roflumilast analysis. The method was validated according to specificity, linearity, LOD, LOQ, accuracy, precision and robustness. 相似文献
42.
Multi‐layered Youden rectangles are introduced. These new designs include double and triple Youden rectangles as subdesigns. Though many double Youden rectangles are known, the triples, introduced in 1994, have to now yielded few nontrivial examples. Two infinite series of multi‐layered Youden rectangles are constructed, and so also many new triple Youden rectangles. 相似文献
43.
G. Durand S. Chiron V. Bouvot D. Barceló 《International journal of environmental analytical chemistry》2013,93(1-2):31-42
Abstract C-18 Empore extraction disks were used for the isolation and trace enrichment of different groups of pesticides from river water and artificial sea water at concentration levels of 0.2, 5 and 20 μg/l [chlorotriazines, (atrazine and simazine), their dealkylated metabolites, (deethyl- and deisopropylatrazine), organophosphorus (parathion-ethyl), phenylurea (linuron), anilide (propanil), carbamate (aldicarb and carbofuran) and carbamate transformation products (aldicarb sulfoxide, aldicarb sulfone and 3-hydroxy-7-phenol carbofuran]. The extraction disks allowed high flow rates thus 51 samples could be processed within 2h. 30 min. For most of the pesticides the recoveries, as determined by liquid chromatography with diode array detection (LC-DAD), varied from 74 up to 125% with coefficients of variations (CV) of 5-10%, whereas for the carbamate transformation products the recoveries were in the range of 30-35% having a CV of 17-21%. At spiking level of 0.2 μg/l the dealkylated triazine metabolites and the carbamate transformation products were not detected at all. 相似文献
44.
In this paper,microphase behavior of an ABC triblock copolymer,polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide),namely PS-b-P2VP-b-PEO,was systematically studied during spin-coating and solvent vapor annealing based on various parameters,including the types of the solvent,spin speed and thickness.The morphological features and the microdomain location of the different blocks were characterized by atomic force microscope (AFM) and high resolution transmission electron microscopy (HRTEM).With increasing thickness,the order-order transition from nanopores array to the pattern of nanostripes was observed due to microdomain coarsening.These processes of pattern transformation were based on the selectivity of toluene for different blocks and on the contact time between solvent molecules and the three blocks.This work provides different templates for preparation of gold nanoparticle array on silicon wafer,which can be adopted as an active surface-enhanced Raman scattering (SERS) substrate for poly(3-hexylthiophene) (P3HT). 相似文献
45.
Huiyun Wu Xin Li Shen Sheng Zhisong Huang Shuhai Huang Siqing Zhao Hua Wang Zhenhai Sun Xiegu Xu Rui Xiao 《Optik》2013
Model of an optical system with coherent laser array source and the piston phase optimized by the stochastic parallel gradient descent algorithm is established. With this model, theory of beam propagation through the optical system in turbulent atmosphere is analyzed, and the analytical formulas of the beam average intensity along the propagation path are derived. Strehl ratio of the received beam induced by intensity disorderly distribution and power efficiency of the received beam are introduced to evaluate performance of the optical system. Under the H-V 5/7 atmospheric turbulent model, performance of an optical system with determinate parameters was calculated, and the influences of the propagation distance and the laser wavelength were numerically analyzed, respectively. 相似文献
46.
《Analytical letters》2012,45(15):2707-2715
Abstract A simple and efficient HPLC analysis method has been developed to simultaneously analyse complicated multi-vitamin preparations, including vitamin B complexes, vitamin C, orotic acid, methionine, calcium pantothenate, nicotinamide, biotin, folic acid and rutin, without using expensive ion pairing reagent. Diode array detector and adjustment of the mobile phase to pH 5.6 are required in the study. This method has been validated with good linearity, reproducibility, recovery and accuracy for analysis of commercial multi-vitamin products. 相似文献
47.
《Analytical letters》2012,45(11):1879-1890
Abstract An efficient and simple high performance liquid chromatography method was developed for determination of Ephedrine. Noscapine, Promethazine and Bromhexine in cough syrup. Cough syrup samples could be cleaned up by anionic exchange solid phase extraction disk. The method has been validated with good recovery, linearity, reproducibility and selectivity for simultaneous determination of common antihistamines in over-the-counter cough syrup mixtures. 相似文献
48.
The traditional microphone configuration used to measure room impulse responses (IRs) according to ISO 3382:2009 is an omnidirectional and figure-8 microphone pair. IRs measurements were taken in a 2500-seat auditorium to determine how the results from a spherical microphone array (an mh acoustics Eigenmike-em32) compare to those from the traditional microphone setup (a Brüel & Kjær Type-4192 omnidirectional microphone and a Sennheiser MKH30 figure-8 microphone). Measurements were obtained at six receiver locations, with three repetitions each in order to first evaluate repeatability. The metrics considered in this study were: reverberation time (T30), early decay time (EDT), clarity index (C80), strength (G), lateral energy fraction (JLF) and late lateral energy level (LJ). Before calculating these quantities, the IRs were filtered to equalize the frequency response of the microphones and sound source. For the spherical array measurements, the omnidirectional (monopole) and figure-8 (dipole) patterns were extracted using beamforming. In terms of repeatability, the average standard deviation of the three measurements at each receiver location averaged across all metrics, receivers, and octave bands was found to be 0.01 just noticeable differences (JNDs). The analysis comparing the measurements from the two microphone configurations yielded differences which were less than 1 JND for the majority of metrics, with a few exceptions of EDT and C80 slightly above 1 JND. Based on this case study, these results indicate that spherical microphone arrays can be used to obtain valid room IR measurements, which will allow for the development of new metrics utilizing the higher spatial resolution made possible with spherical arrays. 相似文献
49.
In this paper, we propose a compensated pixelwise calibration that integrates the effects of the camera housing temperature. The results of this calibration are compared on black body images to classic two points Non Uniformity Correction based calibrations, compensated or not. It is shown that the proposed approach leads to a significant improvement in the thermal resolution with a reduction in the mean error as well as the standard deviation. The approach is finally challenged on a real case measurement focusing on thermoelasticity. The gain in terms of accuracy measurement is highlighted by comparing the proposed calibration to classic calibrations and the scope of interest of this new calibration is discussed. 相似文献
50.
YUAN Jujun LI Hongdong WANG Qiliang CHENG Shaoheng ZHANG Xianke YU Huajun ZHU Xiurong XIE Yingmao 《高等学校化学研究》2014,(1):18-22
TiO2 nanotube(TiNT) arrays were deposited on boron-doped diamond films by a liquid-phase deposition method with ZnO nanorod arrays as the template.The different morphologies of TiNTs have been obtained by controlling the morphology of ZnO template.The X-ray diffraction and energy-dispersive X-ray analysis show that the ZnO nanorod array template has been removed in the TiNTs formation process.The crystalline quality of the TiNTs is improved by increasing the annealing temperature.The band gap of the TiNTs is about 3.25 eV estimated by the UV-Vis absorption spectroscopy,which is close to the value of bulk TiO2.In the photoluminescence spectrum,a broad visible emission in a range of ca.550-750 nm appears due to the surface oxygen vacancies and defects. 相似文献