全文获取类型
收费全文 | 8804篇 |
免费 | 466篇 |
国内免费 | 70篇 |
专业分类
化学 | 5658篇 |
晶体学 | 73篇 |
力学 | 311篇 |
数学 | 1032篇 |
物理学 | 1411篇 |
无线电 | 855篇 |
出版年
2023年 | 75篇 |
2022年 | 296篇 |
2021年 | 288篇 |
2020年 | 259篇 |
2019年 | 322篇 |
2018年 | 317篇 |
2017年 | 227篇 |
2016年 | 419篇 |
2015年 | 297篇 |
2014年 | 438篇 |
2013年 | 877篇 |
2012年 | 522篇 |
2011年 | 554篇 |
2010年 | 413篇 |
2009年 | 365篇 |
2008年 | 415篇 |
2007年 | 350篇 |
2006年 | 346篇 |
2005年 | 253篇 |
2004年 | 252篇 |
2003年 | 230篇 |
2002年 | 239篇 |
2001年 | 112篇 |
2000年 | 151篇 |
1999年 | 99篇 |
1998年 | 71篇 |
1997年 | 74篇 |
1996年 | 74篇 |
1995年 | 60篇 |
1994年 | 63篇 |
1993年 | 67篇 |
1992年 | 57篇 |
1991年 | 51篇 |
1990年 | 37篇 |
1989年 | 53篇 |
1988年 | 54篇 |
1987年 | 39篇 |
1986年 | 29篇 |
1985年 | 54篇 |
1984年 | 39篇 |
1983年 | 29篇 |
1982年 | 38篇 |
1981年 | 46篇 |
1980年 | 34篇 |
1979年 | 23篇 |
1978年 | 27篇 |
1976年 | 23篇 |
1975年 | 23篇 |
1974年 | 20篇 |
1973年 | 19篇 |
排序方式: 共有9340条查询结果,搜索用时 15 毫秒
941.
942.
Salah-ud-din Ghulam Mohi-ud-Din Atif Bin Mansoor Hassan Masood Mustafa Mumtaz 《Signal, Image and Video Processing》2011,5(4):477-483
The ever increasing demand of security has resulted in wide use of Biometric systems. Despite overcoming the traditional verification
problems, the unimodal systems suffer from various challenges like intra class variation, noise in the sensor data etc, affecting
the system performance. These problems are effectively handled by multimodal systems. In this paper, we present multimodal
approach for palm- and fingerprints by feature level and score level fusions (sum and product rules). The proposed multi-modal
systems are tested on a developed database consisting of 440 palm- and fingerprints each of 55 individuals. In feature level
fusion, directional energy-based feature vectors of palm- and fingerprint identifiers are combined to form joint feature vector
that is subsequently used to identify the individual using a distance classifier. In score level fusion, the matching scores
of individual classifiers are fused by sum and product rules. Receiver operating characteristics curves are formed for unimodal
and multimodal systems. Equal Error Rate (EER) of 0.538% for feature level fusion shows best performance compared to score
level fusion of 0.6141 and 0.5482% of sum and product rules, respectively. Multimodal systems, however, significantly outperform
unimodal palm- and fingerprints identifiers with EER of 2.822 and 2.553%, respectively. 相似文献
943.
Jaegoo Lee Judy J. ChaSara Barron David A. MullerR. Bruce van Dover Ebenezer K. AmponsahTuo-Hung Hou Hassan RazaEdwin C. Kan 《Microelectronic Engineering》2011,88(12):3462-3465
We report the fabrication process as well as material and electrical characterization of ultra thin body (UTB) thin film transistors (TFTs) for stackable nonvolatile memories by using in situ phosphorous doped low-temperature polysilicon followed by the chemical mechanical polishing (CMP) process. The resulting polysilicon film is about 13 nm thick with approximately 1019 cm−3 doping. Root mean square surface roughness below 1 nm is achieved. Metal nanocrystals and high-k dielectric are selected for storage nodes and tunneling barriers to achieve low operating voltages. The number density and average diameter of nanocrystals embedded in the gate stack are 7.5 × 1011 cm−2 and 5.8 nm, respectively. Furthermore, scanning transmission electron microscopy (STEM), convergent beam electron diffraction (CBED) and electron energy loss spectroscopy (EELS) are performed for material characterization. The dielectric constant of the (Ti, Dy)xOy film is 35, and the off-state leakage current at −1 V bias and 2.8 nm equivalent oxide thickness is 5 × 10−7 A/cm2. We obtain a memory window of about 0.95 V with ±6 V program/erase voltages. Our results show that UTB TFT is a promising candidate for the three-dimensional integration in high-density nonvolatile memory applications. 相似文献
944.
M. El Kazzi D.J. WebbL. Czornomaz C. RosselC. Gerl M. RichterM. Sousa D. CaimiH. Siegwart J. FompeyrineC. Marchiori 《Microelectronic Engineering》2011,88(7):1066-1069
Experiments to increase the specific capacitance of MOS capacitors consisting of HfO2 on a passivating interfacial layer (IL) of amorphous Si (a-Si) on GaAs are described. XPS analysis of the layers and electrical measurements on the capacitors are combined to study the evolution of the gate stack during deposition and subsequent heat treatments. It is shown that oxidation of the a-Si IL is a major factor in preventing the attainment of a scaled capacitance equivalent thickness (CET). By controlling the deposition of the layers, the gate metal and the heat treatments, a highly scaled gate stack with a CET of 1.2 nm and a leakage reduction of more than 4 orders of magnitude with respect to SiO2/Si was realized. 相似文献
945.
Hydrogenation of π-unsaturated reactants in the presence of carbonyl compounds or imines promotes reductive C-C coupling, providing a byproduct-free alternative to stoichiometric organometallic reagents in an ever-increasing range of C=X (X = O, NR) additions. Under transfer hydrogenation conditions, hydrogen exchange between alcohols and π-unsaturated reactants triggers generation of electrophile-nucleophile pairs, enabling carbonyl addition directly from the alcohol oxidation level, bypassing discrete alcohol oxidation and generation of stoichiometric byproducts. 相似文献
946.
Fatima Bi Seema Iqbal Muhammad Arman Amanat Ali Mahmood-ul Hassan 《Journal of Saudi Chemical Society》2011,15(3):269-273
The effects of polysaccharide elicitor k-carrageenan obtained from Hypnea musciformis, red algae on the production of Induced Secondary Metabolites, ISMs (the disease resistance compounds) and on various growth characters of chickpea and maize plants were studied. Experiments were conducted in the field of PCSIR Laboratories Complex Karachi during December 2008–April 2009 in randomized complete block design with three replications. Three elicitor treatments were used, a solid preparation in which the elicitor was mixed with soil (T2 1 mg/g) and applied around the seeds in the soil. The two other preparations were liquid, T1 and T3 at a concentration of 100 μg glc eq ml−1 and were applied around the sowing seeds and as a foliar spray on the plants, respectively. Statistical analysis of the data revealed that these treatments significantly enhanced all the growth characters of chickpea except T2 that gave the nonsignificant difference in the plant height. Maximum plant height (80.3 cm), number of pods plant−1 (76.2), number of branches plant−1 (25.0), number of leaves plant−1 (125.6), earlier flowering and high ISMs contents in leaves, stem and grains of chickpea were recorded in T1 treated chickpea plants. In maize plants only T1 and T3 treatments (with minor exceptions) had significant effects on few characters like plant height, stem diameter, number of leaves plant−1 and on ISMs contents in leaves while number of cobs plant−1 and flowering time were nonsignificantly affected by these treatments. These results suggested that k-carrageenan elicitor can be used as a potent plant protectant as well as growth promoting agent especially for chickpea plants. 相似文献
947.
Badawi HM Förner W 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(3):1162-1167
The structural stability of phenylacetic acid and mandelic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (Np) forms as their lowest energy structures. The observed spectral intensities of the acids were consistent with the Np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. The sharpness of the methylenic O-H stretching mode in the IR spectrum of mandelic acid suggests the absence of intermolecular dimerization in the acid which is supported by the observation of no splitting of its CO stretching mode. 相似文献
948.
Furans may be ring opened via pallado-catalyzed reactions leading to α,β-unsaturated aldehydes and ketones tethered to indole and isoquinoline moieties. Besides their synthetic interest, these fragmentations bring interesting elements into the discussion around the reaction mechanisms involved in palladium C-H activations of electron-rich heterocycles. 相似文献
949.
The structural stability of 2-phenyl- and 2-phenoxyethanols were investigated at the DFT-B3LYP/6-311G**, MP2 and MP4(SDQ) levels of theory. From the calculations at the three levels of theory 2-phenylethanol and 2-phenoxyethanol were predicted to exist predominantly in non-planar gauche conformations. For 2-phenylethanol the lowest energy Gg1 structure was predicted to be stabilized by an interaction between the hydroxyl H atom and the phenyl ring. For 2-phenoxyethanol the Ggg1 structure was predicted to be strongly stabilized by dipolar interactions between the hydroxyl H atom and the phenoxy O atom of the alcohol. For both alcohols the planar trans structure with minimum steric interactions between the CH2 groups was predicted to be significantly higher in energy than the ground state gauche structure of the alcohols. The dipolar interactions are reported to play more important role than steric ones in stabilizing the molecules. The vibrational frequencies of each of the two alcohols in its lowest energy gauche structure were computed at the B3LYP level and tentative vibrational assignments were made for their normal modes on the basis of the calculated and experimental data. 相似文献
950.
J. A. Stratis M. Lalia-Kantouri El. Charalambous A. Charalambous N. Kantiranis 《Journal of Thermal Analysis and Calorimetry》2011,104(2):431-437
This study is focused on simultaneous thermoanalytical investigations by TG/DTG-DTA technique applied for characterization
of samples collected from archaeological site of Nicosia, Cyprus, dating to seventeenth century and gave new information on
the firing technology. The ceramic samples derived from Ottomanic tobacco pipes were characterized by the related techniques
such as X-ray powder diffraction for the mineralogical composition, and inductively coupled plasma-atomic emission spectrometry
and micro-X-ray fluorescence spectroscopic analysis for the chemical content. It was found that they consisted mainly of quartz,
calcite, feldspars, and micas. For the majority of the investigated ceramic samples, the thermal behavior investigation collaborates
with their mineralogical findings, and resulted to the firing temperature at ~700 °C, due to the existence of calcite. Only
in two samples with very high content in quartz, absence of calcite, low amounts of adsorbed water and of total mass loss,
and absence of micas, the firing process resulted up to 1000 °C. 相似文献