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排序方式: 共有959条查询结果,搜索用时 31 毫秒
61.
We studied the coupled impurity states in a freestanding semiconductor nanowire (NW), within the effective mass approximation and including the effect of the dielectric mismatch, by using finite element method. Bonding and anti-bonding states are found and their energies converge with increasing distance di between the two impurities. The dependence of the binding energy on the wire radius R and the distance di between the two impurities is investigated, and we compare it with the result of a freestanding NW that contains a single impurity. 相似文献
62.
The shape of timber changes due to variations in moisture content (MC). A poor way of designing and/or erecting a structure may result in excessive deformation. An analytical model, which explains spring in a partition wall, is presented. The model is used for parametric studies of various properties such as longitudinal shrinkage coefficient, modulus of elasticity and cross-sectional size of the timber studs and stiffness of the cladding. With respect to spring, a problem is likely to occur when erecting a wall with single-sided cladding in a drying climate and the structure will benefit from relatively stiff studs and weak cladding. 相似文献
63.
A. Magnus G. Carvalho A.A. CoelhoP.J. von Ranke C.S. Alves 《Journal of Alloys and Compounds》2011,509(8):3452-3456
The determination of the isothermal variation of the entropy (ΔST) is discussed in the present work. We show that ΔST has very different profiles and magnitudes when calculated from M vs. H or M vs. T experimental data. For MnAs compound, ΔST obtained from M vs. T data does not present a colossal peak. This result and the agreement between theoretical and experimental non-colossal magnetocaloric effect indicate that the colossal peak may be miscalculated from M vs. H experimental data. For Gd5Ge2Si2 compound, ΔST obtained from M vs. T data does not present the peak observed in ΔST from M vs. H data. 相似文献
64.
Jennifer M. Ullbrand José M. Córdoba Javier Tamayo-Ariztondo María R. Elizalde Mats Nygren Jon M. Molina-Aldareguia Magnus Odén 《Composites Science and Technology》2010
Several types of carbon nanofibres (CNF) were coated with a uniform and dense copper layer by electroless copper deposition. The coated fibres were then sintered by two different methods, spark plasma sintering (SPS) and hot pressing (HP). The Cu coating thickness was varied so that different volume fraction of fibres was achieved in the produced composites. In some cases, the CNF were pre-coated with Cr for the improvement the Cu adhesion on CNF. The results show that the dispersion of the CNF into the Cu matrix is independent of the sintering method used. On the contrary, the dispersion is directly related to the efficiency of the Cu coating, which is tightly connected to the CNF type. Overall, strong variations of the thermal conductivity (TC) of the composites were observed (20–200 W/mK) as a function of CNF type, CNF volume fraction and Cr content, while the coefficient of thermal expansion (CTE) in all cases was found to be considerably lower than Cu (9.9–11.3 ppm/K). The results show a good potential for SPS to be used to process this type of materials, since the SPS samples show better properties than HP samples even though they have a higher porosity, in applications where moderate TC and low CTE are required. 相似文献
65.
Hemmendorff M Andersson MT Kronander T Knutsson H 《IEEE transactions on medical imaging》2002,21(12):1536-1543
We present a method for accurate image registration and motion compensation in multidimensional signals, such as two-dimensional (2-D) X-ray images and three-dimensional (3-D) computed tomography/magnetic resonance imaging volumes. The method is based on phase from quadrature filters, which makes it robust to noise and temporal intensity variations. The method is equally applicable to signals of two, three or higher number of dimensions. We use parametric models, e.g., affine models, finite elements or local affine models with global regularization. Experimental results show high accuracy for 2-D and 3-D motion compensation. 相似文献
66.
Palmblad M Ramström M Markides KE Håkansson P Bergquist J 《Analytical chemistry》2002,74(22):5826-5830
Liquid chromatography coupled on- or off-line with mass spectrometry is rapidly advancing as a tool in proteomics capable of dealing with the inherent complexity in biology and complementing conventional approaches based on two-dimensional gel electrophoresis. Proteins can be identified by proteolytic digestion and peptide mass fingerprinting or by searching databases using short-sequence tags generated by tandem mass spectrometry. This paper shows that information on the chromatographic behavior of peptides can assist protein identification by peptide mass fingerprinting in liquid chromatography/mass spectrometry. This additional information is significant and already available at no extra experimental cost. 相似文献
67.
68.
Synthesis of branched 'nanotrees' by controlled seeding of multiple branching events 总被引:2,自引:0,他引:2
Dick KA Deppert K Larsson MW Mårtensson T Seifert W Wallenberg LR Samuelson L 《Nature materials》2004,3(6):380-384
The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years. Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials. Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour-liquid-solid growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees. 相似文献
69.
Hirschberg D Jägerbrink T Samskog J Gustafsson M Ståhlberg M Alvelius G Husman B Carlquist M Jörnvall H Bergman T 《Analytical chemistry》2004,76(19):5864-5871
A compact disk (CD)-based microfluidic method for selective detection of phosphopeptides by mass spectrometry is described. It combines immobilized metal affinity chromatography (IMAC) and enzymatic dephosphorylation. Phosphoproteins are digested with trypsin and processed on the CD using nanoliter scale IMAC with and without subsequent in situ alkaline phosphatase treatment. This is followed by on-CD matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Dephosphorylation of the IMAC-enriched peptides allows selective phosphopeptide detection based on the differential mass maps generated (mass shifts of 80 Da or multiples of 80 Da). The CD contains 96 microstructures, each with a 16 nL IMAC microfluidic column. Movement of liquid is controlled by differential spinning of the disk. Up to 48 samples are distributed onto the CD in two equal sets. One set is for phosphopeptide enrichment only, the other for identical phosphopeptide enrichment but combined with in situ dephosphorylation. Peptides are eluted from the columns directly into MALDI target areas, still on the CD, using a solvent containing the MALDI matrix. After crystallization, the CD is inserted into a MALDI mass spectrometer for analysis down to the femtomole level. The average success rate in phosphopeptide detection is over 90%. Applied to noncharacterized samples, the method identified two novel phosphorylation sites, Thr 735 and Ser 737, in the ligand-binding domain of the human mineralocorticoid receptor. 相似文献
70.
The deformations around the tip of a straight, through-the-thickness crack in sheets of cellulose nitrate, celluloid, in tension have been studied, as well before as after the onset of stable crack growth. A tensile testing apparatus and a device for engraving a fine grid directly on the specimen sheet have been designed. The development of the plastic enclave also after the start of stable crack extension has been studied in connexion with investigations of the deformations in gross fields of size up to 30 × 40 mm2 performed on a digital computer (line element length 1 or 0.5 mm, virgin crack length 80 mm). Unexpected deformations very close to the crack tip were found in the near field where line elements of length 50 µm were analyzed. A modified Dugdale model where the cohesive stresses vary linearly is examined and is applied on polyvinylehloride, PVC, specimen the thickness of which is strongly reduced near the crack tip. A method for measurement of thickness changes of the order of a few µm by tracing isohypses is described. Finally, to enable comparison with the experimental results, the elastic deformation pattern for a square mesh in the plane strain and plane stress case is obtained by a digital computer.
Zusammenfassung Es wurden die Verformungen an der Spitze eines geraden, über die gesamte Dicke durchgehenden Risses in Spannungen unterworfenen Tafeln aus Zellulosenitrat (Zelluloïd) untersucht. Dies geschah sowohl vor als auch nach der Einstellung einer gleichmäßigen Rißfortpflanzungsgeschwindigkeit. Hierzu wurde eine Zugversuchsapparatur sowie ein Verfahren zur Gravierung eines Rasters, direkt auf die Probe, entwickelt.Die Ausbildung der plastischen Enklave, auch nach Einstellung einer gleichmäßigen Rißausweitung, wurde in Verbindung mit Studien der Verformung in Grobrastern his 30 × 40 mm mittels Digitalrechnern, untersucht (Länge einer Elementarlinie l oder 0,5 mm; Originallänge des Risses 80 mm). Direkt an der Rißspitze, wo Elementarlinien mit einer Länge von 50 µm untersucht wurden, konme man unerwartete Verformungen feststellen. Es wurde ein abgewandeltes Dugdale-Modell bei dem die Kohäsionsspannungen sich linear verändern untersucht and auf Polyvinylchlorid PVC-Proben, deren Dicke sich an der Rißspitze stark reduziert, angewandt. Es wird ein Verfahren zur Messung von Änderungen der Dicke um wenige µm beschrieben. Um einen Vergleich mit den experimentellen Ergebnissen zu ermöglichen, wird zum Schluß mit Hilfe eines Digitalrechners das elastische Verformungsmodell für Quadratraster, im Falle planer Verformung and eines planen Spannungszustandes berechnet.
Résumé On a étudié les déformations qui se produisent au voisinage de l'extrémité d'une fissure droite et traversante dans des feuilles de nitrate de cellulose (celluloid) soumises à traction, et ce avant et après établissement d'un régime stable de propagation. A cet effet, on a mis au point un équipement d'essai de traction et une technique permettant de tracer une grille directement sur l'éprouvette. Le développement d'une enclave plastique dès que l'éxtension de la fissure devient stable a été etudie en recherchant les déformations survenant dans des réseaux grossiers, allant jusqu'à 30 × 40 mm2, a l'áide d'un calculateur digital (longueur d'une ligne élémentaire: 1 ou 0,5 mm, longueur de fissure initiale: 80 mm). Dans un réseau plus fin, où les lignes élémentaires avaient une longueur de 50 microns, on a analysé, au voisinage immédiat de l'éxtrémité de la fissure, des déformations inattendues.On a examiné l'ápplication d'un modèle du Dugdale modifié, où les contraintes de cohésion varient linéairement, à des éprouvettes de PVC, dont l'épaisseur subit des réductions considérables près de l'extrémité de la fissure. On décrit une méthode de mesure des variations d'épaisseur de l'órdre de quelques microns, qui est basée sur le tracé de courbes de niveaux.Enfin, pour permettre une comparaison avec les résultats expérimentaux, on a établi par un calculateur digital l'állure de la déformation élastique dans un réseau carré pour les cas d'état plan de déformation et d'état plan de tension.相似文献