全文获取类型
收费全文 | 3010篇 |
免费 | 334篇 |
国内免费 | 365篇 |
专业分类
化学 | 1170篇 |
晶体学 | 121篇 |
力学 | 68篇 |
综合类 | 26篇 |
数学 | 21篇 |
物理学 | 604篇 |
综合类 | 1699篇 |
出版年
2024年 | 14篇 |
2023年 | 25篇 |
2022年 | 69篇 |
2021年 | 78篇 |
2020年 | 73篇 |
2019年 | 74篇 |
2018年 | 80篇 |
2017年 | 116篇 |
2016年 | 115篇 |
2015年 | 114篇 |
2014年 | 158篇 |
2013年 | 263篇 |
2012年 | 192篇 |
2011年 | 178篇 |
2010年 | 167篇 |
2009年 | 176篇 |
2008年 | 164篇 |
2007年 | 201篇 |
2006年 | 177篇 |
2005年 | 161篇 |
2004年 | 156篇 |
2003年 | 124篇 |
2002年 | 100篇 |
2001年 | 82篇 |
2000年 | 70篇 |
1999年 | 72篇 |
1998年 | 70篇 |
1997年 | 40篇 |
1996年 | 62篇 |
1995年 | 60篇 |
1994年 | 54篇 |
1993年 | 32篇 |
1992年 | 37篇 |
1991年 | 35篇 |
1990年 | 25篇 |
1989年 | 25篇 |
1988年 | 17篇 |
1987年 | 22篇 |
1986年 | 17篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有3709条查询结果,搜索用时 8 毫秒
101.
二氧化碳加氢直接合成二甲醚催化剂的研究 Ⅰ. 沉淀剂对催化剂结构和性能的影响 总被引:21,自引:3,他引:21
使用不同沉淀剂制备了CuO-ZnO-Al2O3/HZSM-5复合型催化剂,考察了其对CO2直接加氢的催化性能,并采用H2-TPR、XRD、BET、CO2-TPD、H2-TPD等表征方法对催化剂物化性质进行了表征。研究结果表明,沉淀剂对催化剂的反应性能、晶相结构、还原难易程度、以及对CO2、H2的吸附性能等均有显著影响。催化剂上存在两种吸附位,催化剂的活性、选择性与其吸附性能密切相关。研究结果可以较好地解释CO2直接转化为甲醇和二甲醚的反应机理。 相似文献
102.
锐钛相虫蛀状介孔二氧化钛的表征 总被引:3,自引:0,他引:3
0IntroductionMesostructuredtitania(TiO2)hasattractedagreatdealofattention,inviewofcontrolleddelivery,catalytic,photocatlytic,orenergyconversionapplica-tions犤1犦.Theuseoftitaniumisopropoxidebis-acetylace-tonateprecursors,combinedwithalkylphosphatean-ionicsurfactantsastemplate,ledtothefirstdocu-mentedpuremesoporousTiO2犤2犦.Insubsequentyears,moresyntheticstrategieshavebeendevelopedusingavarietyofstructure-directingagents,forexample,car-boxylicacids犤3犦,alkylamine犤4犦,aminium犤5犦,blockcop-o… 相似文献
103.
Harald Hillebrecht Natascha Vojteer Vanessa Sagawe Kathrin Hofmann Barbara Albert 《无机化学与普通化学杂志》2019,645(3):362-369
Transparent and nearly colorless single crystals of r‐LiB13C2 were obtained by reaction of boron with Li2CO3 in a Cu melt at 1250–1300 °C. The structure analysis [R3 m, a = 5.6535(1), c = 12.5320(2) Å, 421 independent reflections, 22 parameters, R1 = 0.034, wR2 = 0.093] revealed a crystal structure that can be described as a filling variant of rhombohedral B13C2. Li+ is located in a void above or below the linear CBC unit. The site occupation is close to 50 % resulting in an electron‐precise composition according to Wade's rules if a positive charge is given to the CBC entity: Li+(B12)2–(CBC)+. The displacement parameters of the CBC unit indicate disorder in the [001] direction, that relates to the short Li–C distance and the partial occupation of the Li+ site. The composition is confirmed by EELS measurements of single crystals. Band gap calculations give a value of 2.94 eV, which is in agreement to the crystals being colorless. The evaluation of the electron density by application of the QTAIM formalism as proposed by Bader modifies the assignment pictured above according to Wade's rules. In agreement to the electronegativities the carbon atoms carry a negative charge (–2.31/–2.42) and the effective charges are: Li+0.81(B12)+2.02(CBC)–2.83. 相似文献
104.
目前,均相沉淀法在大学分析化学实验中已很少采用。高锰酸钾间接滴定法测定钙含量是大学分析化学实验中的经典实验之一,历史上存在均相沉淀法和传统沉淀法2种不同的沉淀操作。虽然我国现有大学分析化学实验中已不采用均相沉淀法进行该实验,但在某些分析化学实验教材中,却存在概念混乱的现象,即把传统沉淀法称作均相沉淀法。对这种现象进行了探讨和总结。 相似文献
105.
Dr. Caiyun Geng Prof. Dr. Jilai Li Dr. Thomas Weiske Prof. Dr. Helmut Schwarz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(56):12940-12945
The reactivity of the cationic metal-carbon cluster FeC4+ towards methane has been studied experimentally using Fourier-transform ion cyclotron resonance mass spectrometry and computationally by high-level quantum chemical calculations. At room temperature, FeC4H+ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C−H bond activation step proceeds through a radical-based hydrogen-atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC4+, which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp-orbital of the terminal carbon atom of FeC4+ migrates to the singly occupied π*-orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C−H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed-shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition-metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen-atom transfer. 相似文献
106.
Densification of boron carbide during sintering may be improved by a two-stage process, namely heating to 2000°C under vacuum and sintering at 2190°C under argon. This sintering regime allows achieving a relative density of the ceramic bodies fabricated from a fine powder higher than 95%. The nitrogen treatment of the boron carbide phase at 1900°C leads to the formation of the BN phase and precipitation of graphite. Vacuum treatment of these samples at 2000°C leads to decomposition of the boron nitride phase. The liberated free boron may again react with graphite to form in situ boron carbide particles. The experimental investigations of the sintering behavior of the boron carbide phase under various atmospheres supported the thermodynamic predictions regarding the phase transformation. No evidence, however, was found for enhanced sintering under a nitrogen atmosphere. 相似文献
107.
Preparation, Characterization and Catalytic Properties of S2O8^2-/ZrO2 Supported by Tungsten Carbide
XUEHua-xin CHENJian-min 《高等学校化学研究》2004,20(1):68-72
A WC-supported S2O8^2-/ZrO2(PSZ) catalyst was prepared and characterized by means of XRD, BET, FTIR and XPS. The isomerization of n-pentane over the catalyst was investigated as well. The results show that the skeletal isomerization and the crack of n-pentane proceed simultaneously on WC-supported S2O8^2-/ZrO2 catalyst. The addition of tungsten carbide showed a significant enhancement in the activity and stability of the catalyst for n-pentane isomerization. The catalyst showed evidently a better activity than S2O8^2-/ZrO2 supported by Pt and WO3. The results can be interpreted by the existence of the tungsten oxycarbide compound(WCxOy) with carbidic, oxide and acidic sites. 相似文献
108.
Puoci F Iemma F Muzzalupo R Spizzirri UG Trombino S Cassano R Picci N 《Macromolecular bioscience》2004,4(1):22-26
Spherical molecularly imprinted polymers (SMIPs) have been prepared via a novel precipitation polymerization using sulfasalazine (prodrug used in the diseases of the colon) as template. The sulfasalazine was incorporated into SMIPs and into a spherical non-imprinted polymer (control), and then the release rate of the bioactive agent at different pH values was evaluated. Considerable differences in the release characteristics between imprinted and non-imprinted polymers have been observed. This opens the possibility of the development of drug release systems capable of modulating the release of a specific molecule. Photomicrography of spherical molecularly imprinted polymers (SMIPs). 相似文献
109.
Monodisperse, reactive hydrogel microspheres were prepared by precipitation polymerization ofp-nitrophenyl acrylate (NPA) with acrylamide, methacrylic acid, and methylenebisacrylamide in ethanol. The size of microspheres was controlled by the monomer ratio. Some fraction of reactive ester decomposed during the polymerization. The reactive hydrogel microspheres were converted to amphoteric ones by the reaction of NPA units with diamine. The isoelectric point of the amphoteric microspheres was around 4.0, but it was different from the pH at which the microspheres have the minimum size or the most shrunken state. This was attributed to the uneven distribution of induced amine groups. 相似文献
110.
The ability to recover and purify natural and recombinant proteins, and the costs of doing so remain a major task in introducing
the potential products of biotechnology. The bases for separation range from specific binding onto tailored reagents to solubility
and partitioning behavior governed by a mixed bag of size, charge, and hydrophobicity. In most cases, a combination of methods
is used in sequence, and improvements in the selectivity at an early stage can enhance the effectiveness of subsequent (and
usually more costly) steps. Genetic engineering provides a means of improving the selectivity within the context of existing
separation methods.
By this strategy, improvements in selectivity are sought by bestowing a distinctive property on the protein of interest. The
primary sequence of amino acids is altered, such that the protein can be selectively removed from other components of the
multicomponent mixture in which such products are commonly found. In this article, the range of these “distinctive properties”
and their pairing with various separation methods will be reviewed. Specific examples from our work, in which a distinctive
charge is provided via a polypeptide “purification” fusion tail, will be discussed. Separation methods we have used with these
fusion proteins are precipitation, two-phase aqueous extraction, reversed micellar extraction, and ion exchange using both
resins and membranes. 相似文献