全文获取类型
收费全文 | 2499篇 |
免费 | 433篇 |
国内免费 | 101篇 |
专业分类
化学 | 258篇 |
晶体学 | 15篇 |
力学 | 198篇 |
综合类 | 56篇 |
数学 | 604篇 |
物理学 | 816篇 |
无线电 | 1086篇 |
出版年
2025年 | 11篇 |
2024年 | 51篇 |
2023年 | 46篇 |
2022年 | 66篇 |
2021年 | 69篇 |
2020年 | 70篇 |
2019年 | 62篇 |
2018年 | 45篇 |
2017年 | 90篇 |
2016年 | 86篇 |
2015年 | 91篇 |
2014年 | 134篇 |
2013年 | 175篇 |
2012年 | 148篇 |
2011年 | 152篇 |
2010年 | 135篇 |
2009年 | 148篇 |
2008年 | 186篇 |
2007年 | 150篇 |
2006年 | 155篇 |
2005年 | 136篇 |
2004年 | 86篇 |
2003年 | 106篇 |
2002年 | 98篇 |
2001年 | 75篇 |
2000年 | 72篇 |
1999年 | 56篇 |
1998年 | 54篇 |
1997年 | 35篇 |
1996年 | 34篇 |
1995年 | 30篇 |
1994年 | 30篇 |
1993年 | 24篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 16篇 |
1989年 | 18篇 |
1988年 | 19篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有3033条查询结果,搜索用时 15 毫秒
971.
A systematic study of the dynamic shear modulusG*=G+G in three poly(alkyl methacrylates) (PEMA, PnPMA, PnBMA) at frequencies between 0.001 and 500 rad/s is presented. As the splitting frequencies s are low, aging effects can be observed in the splitting region. There is a systematic shift of the splitting frequency s to lower values with increasing length of the alkyl side group. In PnBMA a separate shear appearance is observed about two frequency decades below the local mode . This is discussed in terms of the concept of minimal cooperativity. Aging effects are: Shift of the maximum loss frequency to lower values, peak sharpening of the relaxation, and intensity changes of and . These effects are discussed in terms of the sequential aging concept. Aging leads to a pronounced bending of the traces upwards from the equilibrium line in the Arrhenius diagram. These non-equilibrium phenomena are promoted by the small slope m=d(log )/dT of the trace in the splitting region.Dedicated to Prof. E. W. Fischer at the occasion of his 65th Birthday Lieber Herr Fischer, die Hallenser Polymerphysiker danken Ihnen aufrichtig für die warmherzige und effektive Förderung der Polymerwissenschaften im Raum Halle-Merseburg. 相似文献
972.
Apoptosis is described as a mechanism of cell death occurring after adequate cellular harm. Deregulation of apoptosis occurs in many human conditions such as autoimmune disorders, ischemic damage, neurodegenerative diseases and different cancer types. Information relating miRNAs to cancer is increasing. miRNAs can affect development of cancer via many different pathways, including apoptosis. Polymorphisms in miRNA genes or miRNA target sites (miRSNPs) can change miRNA activity. Although polymorphisms in miRNA genes are very uncommon, SNPs in miRNA-binding sites of target genes are quite common. Many researches have revealed that SNPs in miRNA target sites improve or decrease the efficacy of the interaction between miRNAs and their target genes. Our aim was to specify miRSNPs on CASP3 gene (caspase-3) and SNPs in miRNA genes targeting 5′UTR and coding exons of CASP3, and evaluate the effect of these miRSNPs and SNPs of miRNA genes with respect to apoptosis. We detected 141 different miRNA binding sites (126 different miRNAs) and 7 different SNPs in binding sites of miRNA in 5′UTR and CDS of CASP3 gene. Intriguingly, miR-339-3p’s binding site on CASP3 has a SNP (rs35372903, G/A) on CASP3 5′UTR and its genomic sequence has a SNP (rs565188493, G/A) at the same nucleotide with rs35372903. Also, miR-339-3p has two other SNPs (rs373011663, C/T rs72631820, A/G) of which the first is positioned at the binding site. Here, miRSNP (rs35372903) at CASP3 5′UTR and SNP (rs565188493) at miR-339-3p genomic sequence cross-matches at the same site of binding region. Besides, miR-339-3p targets many apoptosis related genes (ZNF346, TAOK2, PIM2, HIP1, BBC3, TNFRSF25, CLCF1, IHPK2, NOL3) although it had no apoptosis related interaction proven before. This means that miR-339-3p may also have a critical effect on apoptosis via different pathways other than caspase-3. Hence, we can deduce that this is the first study demonstrating a powerful association between miR-339-3p and apoptosis upon computational analysis. 相似文献
973.
Yue Wang Koki Ogasahara Daisuke Tomihama Radomir Mysliborski Masatoshi Ishida Yongseok Hong Yusuke Notsuka Yoshihisa Yamaoka Tomotaka Murayama Atsuya Muranaka Masanobu Uchiyama Shigeki Mori Yuhsuke Yasutake Susumu Fukatsu Dongho Kim Hiroyuki Furuta 《Angewandte Chemie (International ed. in English)》2020,59(37):16161-16166
The synthesis of organometallic complexes of modified 26π‐conjugated hexaphyrins with absorption and emission capabilities in the third near‐infrared region (NIR‐III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis‐PdII and bis‐PtII complexes of hexaphyrin via N‐confusion modification led to substantial metal dπ–pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO–LUMO energy gap. A remarkable long‐wavelength shift of the lowest S0→S1 absorption beyond 1700 nm was achieved with the bis‐PtII complex, t ‐Pt2‐3 . The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t ‐Pt2‐3 potentially useful as a NIR‐III PA contrast agent. The rigid bis‐PdII complexes, t ‐Pd2‐3 and c ‐Pd2‐3 , are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR‐III‐emissive and photoacoustic‐response capabilities. 相似文献
974.
Single-crystal X-ray structural analysis of K5Na5H3[Nd(α-SiW11O39)2]·6H2O(triclinic, space group P1-, a = 13.7738(10), b = 19.722(2), c = 20.6647(15) (A), α = 111.0090(10), β =105.1740(10), γ = 98-6870(10)°, Z = 2, V = 4867.2(8), Mr = 5914.40, Dc = 4.026, μ = 26.754 mm-1,F(000) = 5076, R = 0.0750 and wR = 0.2077) reveals that two [α-SiW11O39]8- moieties are connected through one Nd atom which is in a square antiprismatic coordination environment by eight oxygen atoms from two [α-SiW11O391]8- moieties. The Nd(Ⅲ) ion is substituted for a [WO]4 unit in the "cap" region of the tungsten-oxygen framework of the parent Keggin ion. Disordered Na cations and water molecules reside outside these channels, leaving empty pores with an intersecting slot (13 (A) × 10 (A)). 相似文献
975.
新疆萨吾尔地区两类花岗岩稀土元素特征 总被引:2,自引:2,他引:2
研究萨吾尔地区两类花岗岩(A型、 I型)的稀土元素特征. 两类花岗岩的稀土元素地球化学特征明显不同, 这与其所处的不同后碰撞构造演化阶段密切相关. 森塔斯岩体和沃肯萨拉岩体为形成于后碰撞阶段晚期挤压-拉张转换体制下的I型花岗岩, 稀土元素特征显示其岩浆演化过程中分离结晶作用不发育, 具幔源特征. 阔依塔斯岩体和恰其海岩体为形成于后碰撞结束阶段拉张体制下的A型花岗岩, 稀土元素特征显示其岩浆演化过程中分离结晶作用发育, 其稀土元素显示的一定的壳源特征应为幔源岩浆受地壳物质混染所致. 相似文献
976.
Yue Wang Koki Ogasahara Daisuke Tomihama Dr. Radomir Mysliborski Dr. Masatoshi Ishida Yongseok Hong Yusuke Notsuka Prof. Dr. Yoshihisa Yamaoka Tomotaka Murayama Dr. Atsuya Muranaka Prof. Dr. Masanobu Uchiyama Dr. Shigeki Mori Dr. Yuhsuke Yasutake Prof. Dr. Susumu Fukatsu Prof. Dr. Dongho Kim Prof. Dr. Hiroyuki Furuta 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(37):16295-16300
The synthesis of organometallic complexes of modified 26π-conjugated hexaphyrins with absorption and emission capabilities in the third near-infrared region (NIR-III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis-PdII and bis-PtII complexes of hexaphyrin via N-confusion modification led to substantial metal dπ–pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO–LUMO energy gap. A remarkable long-wavelength shift of the lowest S0→S1 absorption beyond 1700 nm was achieved with the bis-PtII complex, t -Pt2-3 . The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t -Pt2-3 potentially useful as a NIR-III PA contrast agent. The rigid bis-PdII complexes, t -Pd2-3 and c -Pd2-3 , are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR-III-emissive and photoacoustic-response capabilities. 相似文献
977.
An aromatic diamine monomer,4,4'-bis(3-amino-5-trifluoromethyl phenoxy)-biphenyl (TFBPDA),wassynthesized via the nucleophilic displacement reaction of 3,5-dinitrobenzotrifluoride and 4,4'-biphenol.The monomer wasreacted with various aromatic dianhydrides via the high temperature polycondensation procedure to provide a series ofpolyimides.The polyimides,PI-1 to PI-4,show good solubility not only in aprotic solvents,such as N-methyl-2-pyrrolidinone and N,N-dimethylacetamide,but also in many common solvents,such as m-cresol,chloroform andcyclopentanone.PI-4,derived from 4,4'-(hexafluoroisopropylidene)diphthalic anhydride and TFBPDA,was even soluble intoluene.Moreover,PI films exhibit good thermal stability,outstanding transparency in the visible light region and acceptablemechanical and electrical properties.The excellent combined properties of the polyimides make them as a good candidate forfabricating microelectronics. 相似文献
978.
Performances of fine-grained recognition have been greatly improved thanks to the fast developments of deep convolutional neural networks (DCNN). DCNN methods often treat each image region equally. Besides, researchers often rely on visual information for classification. To solve these problems, we propose a novel discriminative semantic region selection method for fine-grained recognition (DSRS). We first select a few image regions and then use the pre-trained DCNN models to predict their semantic correlations with corresponding classes. We use both visual and semantic representations to represent image regions. The visual and semantic representations are then linearly combined for joint representation. The combination parameters are determined by considering both semantic distinctiveness and spatial-semantic correlations. We use the joint representations for classifier training. A testing image can be classified by obtaining the visual and semantic representations and encoded for joint representation and classification. Experiments on several publicly available datasets demonstrate the proposed method's superiority. 相似文献
979.
Hung Nguyen Manh Ching‐Chun Huang Lee Hsiao‐Yi 《Wireless Communications and Mobile Computing》2016,16(13):1726-1745
During the past decades, many fingerprint‐based indoor positioning systems have been proposed and have achieved great success. However, uncontrolled effects of device diversity, signal noise, and dynamic obstacles could recognizably degrade the performance of modern fingerprint‐based indoor localization systems. In this paper, to amend the variations in radio signal strengths (RSSs) caused by device diversity, we proposed an automatic device calibration process. Because of device diversity, the sensed RSS would deviate from the trained radio map and thus leads to poor positioning. An RSS transform function could be adopted to calibrate the RSS variation between different devices and overcome the device diversity problem. However, to train the transform function, a data collection process is required. Unlike conventional calibration methods requiring manual data collection, we proposed a landmark‐based automatic collection process. Based on the detection of Wi‐Fi landmarks, our system could automatically collect pair‐wise RSS samples between devices and train the RSS transform function without extra human power. In addition, to well represent the effects of signal noise and dynamic obstacles, a region‐based RSS modeling method was also proposed. The proposed modeling method allows our system to perform region‐based target localization and utilize more robust region information for localization. Experiments in various environments demonstrate that our system could give a better positioning performance by properly handling the RSS variation caused by signal noise, dynamic environment, and device diversity. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
980.
Continued scaling of CMOS technology to achieve high performance and low power consumption of semiconductor devices in the complex integrated circuits faces the degradation in terms of electrostatic integrity,short channel effects (SCEs),leakage currents,device variability and reliability etc.Nowadays,multigate structure has become the promising candidate to overcome these problems.SOI FinFET is one of the best multigate structures that has gained importance in all electronic design automation (EDA) industries due to its improved short channel effects (SCEs),because of its more effective gate-controlling capabilities.In this paper,our aim is to explore the sensitivity of underlap spacer region variation on the performance of SOI FinFET at 20 nm channel length.Electric field modulation is analyzed with spacer length variation and electrostatic performance is evaluated in terms of performance parameter like electron mobility,electric field,electric potential,sub-threshold slope (SS),ON current (Ion),OFF current (Ioff) and Ion/Ioff ratio.The potential benefits of SOI FinFET at drain-to-source voltage,VDS =0.05 V and VDS =0.7 V towards analog and RF design is also evaluated in terms of intrinsic gain (Av),output conductance (gd),trans-conductance (gm),gate capacitance (Cgg),and cut-off frequency (fT =gm/2πCgg) with spacer region variations. 相似文献