全文获取类型
收费全文 | 81030篇 |
免费 | 14411篇 |
国内免费 | 5759篇 |
专业分类
化学 | 79496篇 |
晶体学 | 1440篇 |
力学 | 3226篇 |
综合类 | 128篇 |
数学 | 5984篇 |
物理学 | 10926篇 |
出版年
2024年 | 96篇 |
2023年 | 379篇 |
2022年 | 988篇 |
2021年 | 1161篇 |
2020年 | 1967篇 |
2019年 | 3339篇 |
2018年 | 3080篇 |
2017年 | 3588篇 |
2016年 | 4270篇 |
2015年 | 6337篇 |
2014年 | 6211篇 |
2013年 | 8726篇 |
2012年 | 6776篇 |
2011年 | 6628篇 |
2010年 | 5465篇 |
2009年 | 5235篇 |
2008年 | 5673篇 |
2007年 | 5014篇 |
2006年 | 4570篇 |
2005年 | 4237篇 |
2004年 | 3651篇 |
2003年 | 3302篇 |
2002年 | 3717篇 |
2001年 | 2100篇 |
2000年 | 1834篇 |
1999年 | 949篇 |
1998年 | 364篇 |
1997年 | 257篇 |
1996年 | 231篇 |
1995年 | 184篇 |
1994年 | 163篇 |
1993年 | 143篇 |
1992年 | 95篇 |
1991年 | 83篇 |
1990年 | 65篇 |
1989年 | 50篇 |
1988年 | 36篇 |
1987年 | 36篇 |
1986年 | 34篇 |
1985年 | 26篇 |
1984年 | 30篇 |
1983年 | 29篇 |
1982年 | 21篇 |
1981年 | 13篇 |
1980年 | 7篇 |
1979年 | 13篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1972年 | 4篇 |
1971年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
The present short review deals with electroanalytical aspects of electrochemical response of ascorbic acid (Vitamin C) at conducting and electrogenerated polymer modified electrodes. Two main topics are considered: (i) electrocatalytic oxidation of ascorbate at conducting polymer modified electrodes, leading to electroanalytical techniques for ascorbate assay, and (ii) retardation of ascorbate penetration through a layer of electrogenerated polymers, leading to permselective coatings and their diverse uses, especially for biosensing devices. 相似文献
952.
Chiral salicylidenphenethylamines (R)‐HA or (S)‐HA , 2‐salicylidenfurfuryl‐imines HB , and 2‐salicylidenaminoethanol HC react with sodium hydride or sodium hexamethyldisilylamide to form the sodium complexes [Na(R)‐A] 4 · 0,5 Et 2 , [Na(S)‐A] 4 · 0,5 Et 2 O (1) , [NaB] 4 · 0,5 Ph‐Me (2) and [(dme)NaC] 4 (3) . In the presence of 18‐crown‐6 the complex [Na(18‐crown‐6)(thf) 2 ] 2 [Na 2 (C)] 4 · THF (4) can also be isolated. The crystal structure analyses of both 1 and 2 show that heterocubane structures with a Na4O4 frame work are formed. Additionally, the imine nitrogen atom is bonded at the Na atom which has the coordination number 4 in 1 . Additional coordination of the furfuryl oxygen atom results in the coordination number five for the sodium atom in 2 . In 3 which is also a tetramer, two Na2O2 units are connected via two imino‐ethanol bridges Na(1)‐N(=CH‐phenolat)‐CH2CH2‐OH‐Na(2A). The crystal structure analysis displays that 4 is an ionic compound consisting of two [(thf)2Na(18‐crown‐6)]+ cations and the dinuclear dianion [Na 2 (C) 4 ] 2? . Both 1 and 2 are carboxylation reagents which transfer CO2 to 2‐fluoropropiophenone. 1 is more active than 2 , but 3 and 4 are inactive. 相似文献
953.
954.
《Electroanalysis》2004,16(3):224-230
The 3′‐azido‐3′‐deoxythymidine (AZT, Zidovudine) is an antiproliferative and virostatic drug widely used in human immunodeficiency virus type 1 (HIV‐1) infection treatment. With respect to side effects of high doses and a short half‐life of AZT, a fast and simple detection method for this agent could be helpful. The aim of our study was to determine AZT levels in natural samples (urine, serum, whole blood, and cell cultures, such as the HaCaT line of keratinocytes) without their mineralization and/or purification, by means of electrochemical methods using hanging mercury drop electrode (HMDE). On this electrode, AZT undergoes irreversible reduction at the peak potential near Ep?1.1 V (vs. Ag/AgCl/3 M KCl). Reduction AZT signals were measured by cyclic voltammetry (CV), differential pulse voltammetry (DPV), square‐wave voltammetry (SWV), and constant current chronopotentiometric stripping analysis (CPSA). In phosphate buffer (pH 8) the SWV yielded the best AZT signal with the detection limit of 1 nM. The determination of AZT concentration in biological materials is affected by electroactive components, such as proteins and DNA. For monitoring the influence of these compounds, AZT reduction was performed in the presence of 10 μg/mL calf thymus ssDNA and/or 100 μg/mL bovine serum albumin. In these cases, the detection limit increased to 0.25 μM. Also studied was the AZT concentration in keratinocyte cells (HaCaT line) during cell cultivation. It has been shown that the SWV may be considered as a useful tool for the determination of AZT concentration in cell cultures, and for monitoring AZT pharmacokinetics. 相似文献
955.
采用高能球磨法制备了纳米晶掺Y硬质合金粉末。用XRD,SEM和DTA等分析检测手段,研究了纳米晶掺Y硬质合金粉末的结构、形貌和相的变化。结果表明:高能球磨45h,可获得晶粒尺寸约为8nm的掺Y硬质合金粉末;微量Y的加入,有利于硬质合金粉末晶粒的细化;在25~45h范围内,随着高能球磨时间的延长,粉末晶粒尺寸减小,且掺Y硬质合金粉末的晶粒尺寸比未掺Y的硬质合金粉末晶粒尺寸要细一倍;高能球磨25h,粉末中Co的X射线衍射峰消失。高能球磨掺Y硬质合金粉末的DTA曲线在626℃出现了1个尖锐的放热峰。高能球磨掺Y硬质合金粉末固结之后,其合金晶粒细小,机械性能较好。 相似文献
956.
Häussermann U 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(7):1471-1478
The Group 15 elements P, As, Sb, and Bi all have layered structures consisting of six-membered rings under ambient conditions and attain the body-centered cubic (bcc) structure at the highest pressures applied. In the intermediate pressure region, however, phosphorus and its heavier congeners behave profoundly differently. In this region P first attains the open packed simple cubic (sc) structure for a wide range of pressures and then transforms into the rarely observed simple hexagonal (sh) structure. For the heavier congeners complex, incommensurately modulated host-guest structures emerge as intermediate pressure structures. We investigated the high-pressure behavior of P and As by ab initio density functional calculations in which pseudopotentials and a plane wave basis set were employed. The incommensurately modulated high-pressure structure of As was approximated by a supercell. Our calculations reproduced the experimentally established pressure stability ranges of the sc and sh structures for P and the host-guest structure for As very well. We found that the sc and especially the sh structure are decisively stabilized by the admixture of d states in the occupied levels of the electronic structure. This admixture releases s-s antibonding states above the Fermi level (s-d mixing). With pressure, s-d mixing increases rapidly for P, whereas it remains at a low level for As. As a consequence, the band energy contribution to the total energy determines the structural stability for P in the intermediate pressure region, giving rise to simple packed structures. On the other hand, in the intermediate pressure region of the heavier Group 15 elements, a delicate interplay between the electrostatic Madelung energy and the band energy leads to the formation of complex structures. 相似文献
957.
Santiago MB Vélez MM Borrero S Díaz A Casillas CA Hofmann C Guadalupe AR Colón JL 《Electroanalysis》2006,18(6):559-572
We present a carbon paste electrode (CPE) modified using the electron mediator bis(1,10‐phenanthroline‐5,6‐dione)(2,2′‐bipyridine)ruthenium(II) ([Ru(phend)2bpy]2+) exchanged into the inorganic layered material zirconium phosphate (ZrP). X‐Ray powder diffraction showed that the interlayer distance of ZrP increases upon [Ru(phend)2bpy]2+ intercalation from 10.3 Å to 14.2 Å. The UV‐vis and IR spectroscopies results showed the characteristic peaks expected for [Ru(phend)2bpy]2+. The UV‐vis spectrophotometric results indicate that the [Ru(phend)2bpy]2+ concentration inside the ZrP layers increased as a function of the loading level. The exchanged [Ru(phend)2bpy]2+ exhibited luminescence even at low concentration. Modified CPEs were constructed and analyzed using cyclic voltammetry. The intercalated mediator remained electroactive within the layers (E°′=–38.5 mV vs. Ag/AgCl, 3.5 M NaCl) and electrocatalysis of NADH oxidation was observed. The kinetics of the modified CPE shows a Michaelis–Menten behavior. This CPE was used for the oxidation of NADH in the presence of Bakers' yeast alcohol dehydrogenase. A calibration plot for ethanol is presented. 相似文献
958.
A new powerful class of low-molecular-weight amphiphilic compounds has been synthesized and their structure-property relationships with respect to their gelation ability of organic solvents have been investigated. These compounds are able to gel organic solvents over a broad range of polarity. Especially polar solvents such as valeronitrile and gamma-butyrolactone can be gelled even at concentrations far below 1 wt %. It was found that the gelation ability of these asymmetrically substituted p-phenylendiamines depends on a well-balanced relation of the terminal head group, the units involved in hydrogen bonding (amide or urea groups), and on the length of the alkyl chain. With this class of new gelators it is possible to tailor thermal and mechanical properties in different organic solvents and open various application possibilities. 相似文献
959.
The conformational transformation of a 30-residue peptide H(Ala-Gly-Ser-Gly-AIa-Gly)5OH, i.e., (AGSGAG)5, extracted from highly crystalline region of Bombyx mori (B. mori) silk fibroin was described by using the high resolution solid state 13^C NMR, and CD spectroscopies. Based on the conformation-dependent 13^C NMR chemical shifts of the Ala, Gly and Ser residues and the line-shape analysis of the conformation sensitive Ala Cβ resonance, the peptide revealed a strong preference for silk Ⅱ structural form, i,e,, an antiparallel fl-sheet structure (φ= - 140±20°and ψ= 135±20°) in solid state. On the contrary, the CD spectra of this peptide in the two non-native hexafluorinated fibre spinning solvents, hexafluoroisopropanol (HFIP) and hexafluoroacetone (HFA), exhibited the existence of an unusual tightly-folded conformation resembling 310-helix (φ=- 60±20° and ψ=-30±20°), as judged from the R ratio of [θ]222/[θ]203 in HFIP solution, whereas a dynamically averaged unordered structure in HFA, Taken together, the information inclined to hypothesis that the primary structure of the highly crystalline regions of B. mori silk fibroin may be easily accessible to the large conformational changes, which in turn may be critical for facilitating the structural transformation from unprocessed silk fibroin (silk I form) to processed silk fiber (silk Ⅱform). 相似文献
960.
E. Ferrero J. F. Espeso J. G. de la Campa J. de Abajo A. E. Lozano 《Journal of polymer science. Part A, Polymer chemistry》2002,40(21):3711-3724
A series of polyisophthalamides containing pendent phthalimido groups and flexible side spacers were prepared from four novel diacids and three commercial aromatic diamines. These polyamides were prepared in high yields and with high molecular weights by direct polycondensation with triphenyl phosphite and pyridine as condensing agents. The weight‐average and number‐average molecular weights, measured by gel permeation chromatography, were 70,000–137,000 and 47,000–86,000 g/mol, respectively. The novel polyamides were amorphous and readily soluble and showed glass‐transition temperatures of 150–240 °C, as measured by differential scanning calorimetry. Thermogravimetric analysis showed that the 10% weight‐loss temperatures in nitrogen were 355–430 °C, a significant improvement in thermal stability having been observed with the increase in the side‐chain length. A theoretical quantum mechanical study was successfully carried out to explain these results. Flexible and tough films, cast from polymer solutions, showed tensile strengths of 50–125 MPa. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3711–3724, 2002 相似文献