全文获取类型
收费全文 | 620篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
化学 | 426篇 |
晶体学 | 19篇 |
力学 | 7篇 |
数学 | 49篇 |
物理学 | 135篇 |
出版年
2021年 | 10篇 |
2020年 | 8篇 |
2016年 | 7篇 |
2015年 | 8篇 |
2014年 | 11篇 |
2013年 | 30篇 |
2012年 | 18篇 |
2011年 | 18篇 |
2010年 | 17篇 |
2009年 | 11篇 |
2008年 | 27篇 |
2007年 | 20篇 |
2006年 | 23篇 |
2005年 | 23篇 |
2004年 | 23篇 |
2003年 | 8篇 |
2002年 | 19篇 |
2001年 | 14篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 7篇 |
1996年 | 13篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 5篇 |
1990年 | 9篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 5篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1972年 | 7篇 |
1971年 | 8篇 |
1970年 | 7篇 |
1965年 | 5篇 |
1915年 | 5篇 |
1913年 | 5篇 |
1909年 | 10篇 |
排序方式: 共有636条查询结果,搜索用时 15 毫秒
21.
The separation and detection of commonly used preservatives (benzoate, sorbate) and vitamin C by both conventional CE and microchip electrophoresis with capacitively coupled contactless conductivity detection is presented. The separation was optimized by adjusting the pH-value of the buffer and the use of hydroxypropyl-beta-CD (HP-beta-CD) and CTAB as additives. For conventional CE, optimal separation conditions were achieved in a histidine/tartrate buffer at pH 6.5, containing 0.025% HP-beta-CD and 0.1 mM CTAB. LOD ranged from 0.5 to 3 mg/L (S/N = 3) and the RSDs for migration time and peak area were less than 0.1 and 2%, respectively. A considerable reduction of analysis time can be accomplished by using microchip electrophoresis without significant loss in sensitivity under optimal separation conditions. A histidine/tartrate buffer at pH 6.5, incorporating 0.06% HP-beta-CD and 0.25 mM CTAB, gave detection limits ranging between 3 and 10 mg/L and satisfactory reproducibilities of < or =0.4% for the migration time and < or =3.5% for the peak area. The methods developed are useful for the quantitative determination of food additives in real samples such as soft drinks and vitamin C tablets. 相似文献
22.
Köhler K Förster G Hauser A Dobner B Heiser UF Ziethe F Richter W Steiniger F Drechsler M Stettin H Blume A 《Journal of the American Chemical Society》2004,126(51):16804-16813
The temperature-dependent self-assembly of the single-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(trimethylammonio)ethyl phosphate] (PC-C32-PC) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray scattering, rheological measurements, and dynamic light scattering (DLS). At room temperature this compound, in which two phosphocholine headgroups are connected by a C(32) alkyl chain, proved to be capable of gelling water very efficiently by forming a dense network of nanofibers (Kohler et al. Angew. Chem., Int. Ed. 2004, 43, 245). A specific feature of this self-assembly process is that it is not driven by hydrogen bonds but solely by hydrophobic interactions of the long alkyl chains. The nanofibers have a thickness of roughly the molecular length and show a helical superstructure. A model for the molecular structure of the fibrils which considers the extreme constitution of the bolaamphiphile is proposed. Upon heating the suspensions three different phase transitions can be detected. Above 49 degrees C, the temperature of the main transition where the alkyl chains become "fluid", a clear low-viscosity solution is obtained due to a breakdown of the fibrils into smaller aggregates. Through mechanical stress the gel structure can be destroyed as well, indicating a low stability of these fibers. The gel formation is reversible, but as a drastic rearrangement of the molecules takes place, metastable states occur. 相似文献
23.
Melek Gul Dincmen Peter J. Hauser Nevin Cigdem Gursoy 《Plasma Chemistry and Plasma Processing》2016,36(5):1377-1391
In this study, three hydrophilic monomers; 2-acrylamido-2-methyl-1-propanesulfonic acid, 2-hydroxyethyl methacrylate, diallyldimethylammonium chloride (DADMAC) were selected and their performance as an antistatic finish on nylon 6,6 fabrics was investigated. A non-thermal, high density atmospheric pressure plasma was used to graft polymerize the monomers on nylon 6,6 fabrics. Fabrics were first treated with solutions of monomer in water, air dried and then treated with helium plasma to graft polymerize the monomer on the fiber surface. Surface resistivity values were measured before and after soxhlet extraction with water. Results showed that the DADMAC monomer provided better antistatic properties to fabrics. Further studies with DADMAC monomer were made; effects of plasma post exposure time, plasma pre-exposure time, plasma power, concentration of the monomer and existence of a crosslinker were investigated. Higher plasma power, higher concentration of the monomer and longer post exposure times all gave better antistatic properties to the nylon 6,6 fabrics. Acid dye staining, UV–Vis and FT-IR measurements were conducted and results confirmed a grafted poly-DADMAC layer on the fabric surface. 相似文献
24.
25.
C. Tschierske D. Joachimi H-M. Vorbrodt H. Zaschke A. Wiegeleben A. Hauser 《Liquid crystals》2013,40(1):177-190
Abstract A wide variety of new sulphur-containing heteroalicyclic liquid-crystalline compounds have been synthesized. Special attention has thus been focussed on the influence of the heteroatoms on the mesomorphic behaviour. 2-Cyclohexyl substituted 1,3-dithianes and 1,3-oxathianes have been found to be superior to those with 2-phenyl substituents. The clearing temperatures of the 2-phenyl-1,3-dithianes depend on the electron donating or electron accepting properties of the substituents attached to the phenyl group. The mesophase stability of the 2-cyclohexyl substituted (hetero)alicycles increases in the order: 1,3-dioxane < 1,3-oxathiane < cyclohexane < 1,3-dithiane. This order is partly reversed for the cyclohexane annulated compounds: 1-oxa-3-thiadecalin < 1,3-dioxadecalin < decalin < 1,3-dithiadecalin. Selected values of the birefrigence and some melting enthalpies have been measured. 相似文献
26.
Thanh Duc Mai Benjamin Bomastyk Hong Anh Duong Hung Viet Pham Peter C. Hauser 《Analytica chimica acta》2012
An extension of a capillary electrophoresis instrument coupled to a sequential injection analysis manifold was developed for automated measurements with on-line solid-phase extraction preconcentration. An in-house built capacitively coupled contactless conductivity detector was employed for sensitive detection with narrow capillaries of 25 μm internal diameter. The system was assembled into standardized 19 in. frames and racks for easy transport and mobile deployment. The system can be left running unattendedly without manual intervention with good operation stability. To demonstrate the application of the system, a method for the determination of four drugs, namely ibuprofen, diclofenac, naproxen and bezafibrate, was developed with enrichment factors of up to several hundreds. Detection of the drug residues down to the nM-scale was found possible and the method was found suitable for the detection of ibuprofen in the waste water of a hospital in Hanoi. 相似文献
27.
Dr. William Herzog Dr. Catherine Bronner Susanne Löffler Dr. Bice He Daniel Kratzert Prof. Dietmar Stalke Prof. Andreas Hauser Prof. Oliver S. Wenger 《Chemphyschem》2013,14(6):1168-1176
Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal‐to‐ligand charge‐transfer excited state of the [Re(CO)3(phen)(py)]+ complex (phen=1,10‐phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady‐state and time‐resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited‐state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton‐coupled electron‐transfer process. 相似文献
28.
Sazlinda Kamaruzaman Peter C. Hauser Mohd Marsin Sanagi Wan Aini Wan Ibrahim Salasiah Endud Hong Heng See 《Analytica chimica acta》2013
A simple adsorption/desorption procedure using a mixed matrix membrane (MMM) as extraction medium is demonstrated as a new miniaturized sample pretreatment and preconcentration technique. Reversed-phase particles namely polymeric bonded octadecyl (C18) was incorporated through dispersion in a cellulose triacetate (CTA) polymer matrix to form a C18-MMM. Non-steroidal anti-inflammatory drugs (NSAIDs) namely diclofenac, mefenamic acid and ibuprofen present in the environmental water samples were selected as targeted model analytes. The extraction setup is simple by dipping a small piece of C18-MMM (7 mm × 7 mm) in a stirred 10 mL sample solution for analyte adsorption process. The entrapped analyte within the membrane was then desorbed into 100 μL of methanol by ultrasonication prior to high performance liquid chromatography (HPLC) analysis. Each membrane was discarded after single use to avoid any analyte carry-over effect. Several important parameters, such as effect of sample pH, salting-out effect, sample volume, extraction time, desorption solvent and desorption time were comprehensively optimized. The C18-MMM demonstrated high affinity for NSAIDs spiked in tap and river water with relative recoveries ranging from 92 to 100% and good reproducibility with relative standard deviations between 1.1 and 5.5% (n = 9). The overall results obtained were found comparable against conventional solid phase extraction (SPE) using cartridge packed with identical C18 adsorbent. 相似文献
29.
Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B2 and B4 phases. As usual, two ranges of relaxation were detected in the B2 phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B4 phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B2 modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B4 phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B4 phase; then do not appear in the crystalline state. For this reason the B4 phase is regarded as different from a classical crystalline phase. 相似文献
30.
Ingrid Hauser Ana B. Colaço Julie A. Skjæran Aslak Einbu Kjetill Østgaard Hallvard F. Svendsen Francisco J. Cervantes 《Applied biochemistry and biotechnology》2013,169(4):1449-1458
Large-scale amine-based CO2 capture will generate waste containing large amounts of ammonia, in addition to contaminants such as the actual amine as well as degradation products thereof. Monoethanolamine (MEA) has been a dominant amine applied so far in this context. This study reveals how biological N removal can be achieved even in systems heavily contaminated by MEA in post- as well as pre-denitrification treatment systems, elucidating the rate-limiting factors of nitrification as well as aerobic and denitrifying biodegradation of MEA. The hydrolysis of MEA to ammonia readily occurred both in post- and pre-denitrification treatment systems with a hydraulic retention time of 7 h. MEA removal was ≥99?±?1 % and total nitrogen removal 77?±?10 % in both treatment systems. This study clearly demonstrates the advantage of pre-denitrification over post-denitrification for achieving biological nitrogen removal from MEA-contaminated effluents. Besides the removal of MEA, the removal efficiency of total nitrogen as well as organic matter was high without additional carbon source supplied. 相似文献