全文获取类型
收费全文 | 2538篇 |
免费 | 131篇 |
国内免费 | 16篇 |
专业分类
化学 | 1983篇 |
晶体学 | 23篇 |
力学 | 82篇 |
数学 | 116篇 |
物理学 | 481篇 |
出版年
2024年 | 4篇 |
2023年 | 18篇 |
2022年 | 72篇 |
2021年 | 71篇 |
2020年 | 43篇 |
2019年 | 60篇 |
2018年 | 41篇 |
2017年 | 42篇 |
2016年 | 95篇 |
2015年 | 93篇 |
2014年 | 119篇 |
2013年 | 159篇 |
2012年 | 270篇 |
2011年 | 255篇 |
2010年 | 153篇 |
2009年 | 136篇 |
2008年 | 189篇 |
2007年 | 157篇 |
2006年 | 146篇 |
2005年 | 124篇 |
2004年 | 100篇 |
2003年 | 93篇 |
2002年 | 87篇 |
2001年 | 36篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 8篇 |
1997年 | 8篇 |
1996年 | 5篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有2685条查询结果,搜索用时 15 毫秒
91.
We grew heterojunction light emitting diode (LED) structures with various n-type semiconducting layers by magnetron sputtering on p-type GaN at high temperature. Because the undoped ZnO used as an active layer was grown under oxygen rich atmosphere, all LED devices showed the EL characteristics corresponding to orange-red wavelength due to high density of oxygen interstitial, which was coincident with the deep level photoluminescence emission of undoped ZnO. The use of the Ga doped layers as a top layer provided the sufficient electron carriers to active region and resulted in the intense EL emission. The LED sample with small quantity of Mg incorporated in MgZnO as an n-type top layer showed more intense emission than the LED with ZnO, in spite of the deteriorated electrical and structural properties of the MgZnO film. This might be due to the improvement of output extraction efficiency induced by rough surface. 相似文献
92.
We study an optimal portfolio and consumption choice problem of a family that combines life insurance for parents who receive deterministic labor income until the fixed time T. We consider utility functions of parents and children separately and assume that parents have an uncertain lifetime. If parents die before time T, children have no labor income and they choose the optimal consumption and portfolio with remaining wealth and life insurance benefit. The object of the family is to maximize the weighted average of utility of parents and that of children. We obtain analytic solutions for the value function and the optimal policies, and then analyze how the changes of the weight of the parents’ utility function and other factors affect the optimal policies. 相似文献
93.
We prove that the MacWilliams duality holds for bent functions. It enables us to derive the concept of formally self-dual
Boolean functions with respect to their near weight enumerators. By using this concept, we prove the Gleason-type theorem
on self-dual bent functions. As an application, we provide the total number of (self-dual) bent functions in two and four
variables obtaining from formally self-dual Boolean functions. 相似文献
94.
Hyun Woo Nho Jong Yun Kim Jian Wang Hyun‐Joon Shin Sung‐Yool Choi Tae Hyun Yoon 《Journal of synchrotron radiation》2014,21(1):170-176
Here, an in situ probe for scanning transmission X‐ray microscopy (STXM) has been developed and applied to the study of the bipolar resistive switching (BRS) mechanism in an Al/graphene oxide (GO)/Al resistive random access memory (RRAM) device. To perform in situ STXM studies at the C K‐ and O K‐edges, both the RRAM junctions and the I0 junction were fabricated on a single Si3N4 membrane to obtain local XANES spectra at these absorption edges with more delicate I0 normalization. Using this probe combined with the synchrotron‐based STXM technique, it was possible to observe unique chemical changes involved in the BRS process of the Al/GO/Al RRAM device. Reversible oxidation and reduction of GO induced by the externally applied bias voltages were observed at the O K‐edge XANES feature located at 538.2 eV, which strongly supported the oxygen ion drift model that was recently proposed from ex situ transmission electron microscope studies. 相似文献
95.
Esen S Schwartz AJ Adachi I Aihara H Arinstein K Aulchenko V Aushev T Aziz T Bakich AM Balagura V Barberio E Bay A Bischofberger M Bondar A Bozek A Bračko M Browder TE Chang MC Chang P Chen A Chen P Cheon BG Chiang CC Choi Y Dalseno J Dash M Doležal Z Drásal Z Drutskoy A Eidelman S Goldenzweig P Golob B Ha H Haba J Hara T Hayasaka K Higuchi T Hoshi Y Hou WS Hsiung YB Hyun HJ Iijima T Inami K Itoh R Iwabuchi M Joshi NJ Julius T Kang JH Kawasaki T Kichimi H Kim HJ Kim HO Kim JH Kim YJ Kinoshita K 《Physical review letters》2010,105(20):201802
We have made the first observation of B(s)(0)→D(s)(*)+ D(s)(*)- decays using 23.6 fb(-1) of data recorded by the Belle experiment running on the Υ(5S) resonance. The branching fractions are measured to be B(B(s)(0)→D(s)+ D(s)-)=(1.03(-0.32-0.25)(+0.39+0.26))%, B(B(s)(0)→D(s)(*±) D(s)(?))=(2.75(-0.71)(+0.83)±0.69)%, and B(B(s)(0)→D(s)*+ D(s)*-)=(3.08(-1.04-0.86)(+1.22+0.85))%; the sum is B[B(s)(0)→D(s)(*)+ D(s)(*)-]=(6.85(-1.30-1.80)(+1.53+1.79))%. Assuming B(s)(0)→D(s)(*)+ D(s)(*)- saturates decays to CP-even final states, the branching fraction determines the ratio ΔΓ(s)/cosφ, where ΔΓ(s) is the difference in widths between the two B(s)-B(s) mass eigenstates, and φ is a CP-violating weak phase. Taking CP violation to be negligibly small, we obtain ΔΓ(s)/Γ(s)=0.147(-0.030)(+0.036)(stat)(-0.041)(+0.042)(syst), where Γ(s) is the mean decay width. 相似文献
96.
The carbamoylphosphosphonate silane (CMPO analogue; 2-(diphenylphosphoryl)-N-(3-(triethoxysilyl)propyl) acetamide) modified mesoporous silica was prepared via a post-synthesis grafting method for the effective purification of rare earth elements. The guest CMPO analogue was synthesized by direct coupling reaction of 2-(diphenylphosphoryl) acetic acid and 3-(triethoxysilyl)propan-1-amine. Various mesoporous silicates such as MCM-41, SBA-15, or amorphous silica nanoparticles were adopted as host materials. The resulting surface-modified mesoporous materials were characterized with respect to their structural integrity, surface area, and pore size and the concentration of the CMPO silane species. These CMPO functionalized periodic mesostructured silicates offer the potential of applications as catalysts, sensors, or environmental sorbents. 相似文献
97.
Jang Hyun Jo 《代数通讯》2013,41(5):1577-1587
In case G is a finite group, there is a well-known criterion for projective modules: A ? G-module M is projective if and only if it is ? -free and has finite projective dimension. We first investigate whether only finite groups satisfy the above criterion. In the class of groups L H 𝔉, we conclude that this is true. Secondly, we consider the problem when a stably flat Γ-module is projective, where Γ is an arbitrary group. We show that if Γ is an L H 𝔉-group, then every stably flat cofibrant ? Γ-module is projective. 相似文献
98.
Jong Yoon Hyun 《Designs, Codes and Cryptography》2009,50(2):173-185
We present generalized MacWilliams identities for binary codes. These identities naturally lead to the concepts of the local
weight distribution of a binary code with respect to a word u and its MacWilliams u-transform. In the case that u is the all-one word, these ones correspond to the weight distribution of a binary code and its MacWilliams transform, respectively.
We identify a word v with its support, and consider v as a subset of {1, 2,..., n}. For two words u,w of length n such that their intersection is the empty set, define the u-face centered at w to be the set . A connection between our MacWilliams u-transform and the weight distribution of a binary code in the u-face centered at the zero word is presented. As their applications, we also investigate the properties of a perfect binary
code. For a perfect binary code C, the main results are as follows: first, it is proved that our local weight distribution of C is uniquely determined by the number of codewords of C in the orthogonal u-face centered at the zero word. Next, we give a direct proof for the known result, concerning the weight distribution of
a coset of C in the u-face centered at the zero word, by A. Y. Vasil’eva without using induction. Finally, it is proved that the weight distribution
of C in the orthogonal u-face centered at w is uniquely determined by the codewords of C in the u-face centered at the zero word.
相似文献
99.
Tae Hee Kim Hyun Mo Lee Hee Soo Park Sung Dong Kim Su Jin Kwon Atsushi Tahara Hideo Nagashima Bun Yeoul Lee 《应用有机金属化学》2019,33(4)
The original Sasol catalytic system for ethylene tetramerization is composed of a Cr source, a PNP ligand, and MAO (methylaluminoxane). The use of expensive MAO in excess has been a critical concern in commercial operation. Many efforts have been made to replace MAO with non‐coordinating anions (e.g., [B(C6F5)4]?); however, most of such attempts were unsuccessful. Herein, an extremely active catalytic system that avoids the use of MAO is presented. The successive addition of two equivalent [H(OEt2)2]+[B(C6F5)4]? and one equivalent CrCl3(THF)3 to (acac)AlEt2 and subsequent treatment with a PNP ligand [CH3(CH2)16]2C(H)N(PPh2)2 ( 1 ) yielded a complex presumably formulated as [ 1 ‐CrAl (acac)Cl3(THF)]2+[B(C6F5)4]?2, which exhibited high activity when combined with iBu3Al (1120 kg/g‐Cr/h; ~4 times that of the original Sasol system composed of Cr (acac)3, iPrN(PPh2)2, and MAO). Via the introduction of bulky trialkylsilyl substituents such as –SiMe3, –Si(nBu)3, or –SiMe2(CH2)7CH3 at the para‐position of phenyl groups in 1 (i.e., by using [CH3(CH2)16]2C(H)N[P(C6H4‐p‐SiR3)2]2 instead of 1 ), the activities were dramatically improved, i.e., tripled (2960–3340 kg/g‐Cr/h; more than 10 times that of the original Sasol system). The generation of significantly less PE (<0.2 wt%) even at a high temperature is another advantage achieved by the introduction of bulky trialkylsilyl substituents. NMR studies and DFT calculations suggest that increase of the steric bulkiness on the alkyl‐N and P‐aryl moieties restrict the free rotation around (alkyl)N–P (aryl) bonds, which may cause the generation of more robust active species in higher proportion, leading to extremely high activity along with the generation of a smaller amount of PE. 相似文献
100.
Jiwon Choi Jun Seop Yun Hyeeun Song Yong-Keol Shin Young-Hoon Kang Palinda Ruvan Munashingha Jeongyeon Yoon Nam Hee Kim Hyun Sil Kim Jong In Yook Dongseob Tark Yun-Sook Lim Soon B. Hwang 《Molecules (Basel, Switzerland)》2021,26(12)
African swine fever virus (ASFV) causes a highly contagious and severe hemorrhagic viral disease with high mortality in domestic pigs of all ages. Although the virus is harmless to humans, the ongoing ASFV epidemic could have severe economic consequences for global food security. Recent studies have found a few antiviral agents that can inhibit ASFV infections. However, currently, there are no vaccines or antiviral drugs. Hence, there is an urgent need to identify new drugs to treat ASFV. Based on the structural information data on the targets of ASFV, we used molecular docking and machine learning models to identify novel antiviral agents. We confirmed that compounds with high affinity present in the region of interest belonged to subsets in the chemical space using principal component analysis and k-means clustering in molecular docking studies of FDA-approved drugs. These methods predicted pentagastrin as a potential antiviral drug against ASFVs. Finally, it was also observed that the compound had an inhibitory effect on AsfvPolX activity. Results from the present study suggest that molecular docking and machine learning models can play an important role in identifying potential antiviral drugs against ASFVs. 相似文献