全文获取类型
收费全文 | 74033篇 |
免费 | 15866篇 |
国内免费 | 67篇 |
专业分类
电工技术 | 1136篇 |
综合类 | 31篇 |
化学工业 | 23957篇 |
金属工艺 | 1101篇 |
机械仪表 | 1464篇 |
建筑科学 | 3634篇 |
矿业工程 | 117篇 |
能源动力 | 1915篇 |
轻工业 | 9991篇 |
水利工程 | 715篇 |
石油天然气 | 172篇 |
武器工业 | 5篇 |
无线电 | 9289篇 |
一般工业技术 | 17467篇 |
冶金工业 | 7597篇 |
原子能技术 | 299篇 |
自动化技术 | 11076篇 |
出版年
2023年 | 218篇 |
2022年 | 258篇 |
2021年 | 922篇 |
2020年 | 1876篇 |
2019年 | 3718篇 |
2018年 | 3822篇 |
2017年 | 4077篇 |
2016年 | 4633篇 |
2015年 | 4609篇 |
2014年 | 4857篇 |
2013年 | 7326篇 |
2012年 | 4345篇 |
2011年 | 4455篇 |
2010年 | 4279篇 |
2009年 | 4074篇 |
2008年 | 3905篇 |
2007年 | 3691篇 |
2006年 | 3272篇 |
2005年 | 2841篇 |
2004年 | 2545篇 |
2003年 | 2452篇 |
2002年 | 2331篇 |
2001年 | 1819篇 |
2000年 | 1645篇 |
1999年 | 1089篇 |
1998年 | 759篇 |
1997年 | 719篇 |
1996年 | 634篇 |
1995年 | 627篇 |
1994年 | 583篇 |
1993年 | 558篇 |
1992年 | 533篇 |
1991年 | 313篇 |
1990年 | 449篇 |
1989年 | 406篇 |
1988年 | 338篇 |
1987年 | 374篇 |
1986年 | 335篇 |
1985年 | 436篇 |
1984年 | 429篇 |
1983年 | 331篇 |
1982年 | 311篇 |
1981年 | 302篇 |
1980年 | 278篇 |
1979年 | 276篇 |
1978年 | 251篇 |
1977年 | 236篇 |
1976年 | 223篇 |
1975年 | 198篇 |
1974年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ranjit Prasad Vanja Pasanovic‐Zujo Rahul K. Gupta Ferenc Cser Sati N. Bhattacharya 《Polymer Engineering and Science》2004,44(7):1220-1230
Shear and extensional rheological measurements were conducted in conjunction with laser light scattering (LLS) on ethylene‐vinyl acetate copolymer (EVA) nanocomposites. The materials were prepared by melt‐mixing EVA and commercially acquired layered silicates. Wide Angle X‐Ray Scattering (WAXS) was used to ascertain the degree of layer swelling. This could be attributed to the intercalation of polymer chains into the interlayer of the silicates. The nanocomposites prepared were determined to be predominantly intercalated in nature. In shear rheological tests, the nanocomposites exhibited an increase in viscoelastic properties compared to the pure EVA. The extent of this property enhancement was not as pronounced as had been reported in many instances with respect to other polymer nanocomposites. This could be attributed to the absence of a network structure normally observed in an exfoliated system. The extensional rheological tests showed an increase in extensional flow properties. This was confirmed by the LLS, which indicated that the filled systems had higher deformability than the unfilled one. Polym. Eng. Sci. 44:1220–1230, 2004. © 2004 Society of Plastics Engineers. 相似文献
992.
The effects of poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), sodium polyacrylate (NaPA), poly(ethylene glycol) (PEG), pectin (P), and carboxymethyl cellulose (CMC) on the corrosion of cadmium in a 0.5M hydrochloric acid (HCl) solution were studied with both electrochemical impedance spectroscopy and Tafel plot techniques. Measurements were carried out at cathodic, open‐circuit, and anodic potentials. All the investigated polymers had inhibitory effects on both the cathodic (except for NaPA, P, and CMC) and anodic processes, with a predominant anodic inhibiting action. However, NaPA, P, and CMC exhibited a slight cathodic inhibiting action only at higher polymer concentrations. This behavior may be attributed to the very weak adsorbability of the polymers on the cathodic sites. Because PVA and PEG had hydroxy groups, there could be bridging between the polymer and the surface, resulting in an inhibiting effect in the HCl solution. However, PVA had much greater adsorbability on the surface than PEG at the anodic potential. The adsorption of most of the polymers obeyed a Temkin adsorption isotherm, and this indicated indicating that the main process of inhibition was adsorption. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 866–871, 2003 相似文献
993.
A case study was carried out to investigate the bed agglomeration observed in a fluidized bed incinerator when burning blends of three wastes (carbon soot, biosludge and fuel oil). Several instrumental approaches were employed (i.e. XRF, SEM, XRD, and ICP-AES) to identify the bed materials (fresh sand and degrader sand) and clinkers formed in the full-scale incinerator tests. Several elements (V, Al, S, Na, Fe, Ni, P, and Cl), which normally are associated with the formation of low melting point compounds, were found in the waste blends at high content levels. The clinker bridges were identified to be associated with Al, Fe, V, K, Na, S, Ni, and Si elements.The effects of temperature and blending ratio were investigated in a muffle furnace. Carbon soot is believed to be more susceptible to the clinker formation than the other two fuels. Thermodynamic multi-phase multi-component equilibrium calculations predict that the main low melting point species could be Al2(SO4)3, Fe2(SO4)3, Na2SO4, NaCl, Na2SiO3 and V2O5. This information is useful to understand the chemistry of clinker formation. Also, it helps to develop methods for the control and possible elimination of the agglomeration problem for the design fuels. 相似文献
994.
Methyl 8-[2-(cis-pent-2′-enyl)-3-oxo-cis-cyclopent-4-enyl] octanoate (I) is the methyl ester of a cyclic fatty acid synthesized enzymically from an incubation of
linolenic acid with an extract of flaxseed (Linum usitatissimum L.). A proposed trivial name for I is methyl 12-oxo-cis-10, 15-phytodienoate (12-oxo-PDA). The evidence presented indicated that compound I has thecis configuration of the carbon chains with respect to the cyclopentenone ring. Treatment with acid, base, or heat isomerized
I to a second product (II) that has thetrans configuration of the carbon chains. Prolonged heat treatment of II yielded a third cyclic product, methyl 12-oxo-9(13),cis-15-phytodienoate (III). 相似文献
995.
The impact that some membrane preparation steps had on ultrafiltration (UF) membrane characteristics and performance was studied. Polyethersulfone (PES) was employed as base polymer, while N‐methyl pyrrolidone (NMP) was used as a solvent, and polyvinylpyrrolidone (PVP) was used as a nonsolvent pore‐forming additive. The manufacturing variables studied were solvent evaporation time and membrane surface modification, using a fluorine‐based copolymer referred to as surface‐modifying macromolecule (SMM). The flat sheet membranes, prepared via phase inversion, were characterized using solute transport data, X‐ray photoelectron spectroscopy (XPS), and contact angle measurements. Membrane performance was evaluated via filtration test protocol that included a 6‐day filtration of concentrated river water. The flux reduction with time was modeled using single and dual mechanisms of fouling. The pore blockage/cake filtration model described better the behavior of the permeation rate along the experiments. Increasing the solvent evaporation time decreased the size of the pores and the permeation rate. However, it did not significantly affect the removal of the organic compounds naturally present in the river water used as feed. XPS and contact angle measurements proved that the short evaporation periods did not allow enough SMM migration to the surface to provoke a significant effect on the membrane performance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006 相似文献
996.
The transitions and reactions involved in the thermal processing of binary mixtures of polyethylene and poly(ethylene‐co‐vinyl acetate) copolymers with different concentrations of a foaming agent (azodicarbonamide) were studied using differential scanning calorimetry (DSC). The effect of ZnO as a kicker also was discussed. The temperature at the maximum rate and the heat evolved were measured for all the processes—melting, transitions, and reactions—all the mixtures prepared were measured and compared. Azodicarbonamide decomposed differently depending on the polymeric matrix. These data can be very useful for the plastic processing industry. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2015–2025, 2006 相似文献
997.
Ti‐based Ziegler–Natta catalysts supported on MgCl2 doped with AlCl3 were prepared by the reaction of MgCl2/AlCl3–ethanol adduct with TiCl4. No AlCl3 crystallites were found in the AlCl3‐doped catalysts by WAXD analysis, suggesting that AlCl3/MgCl2 solid solution was formed. The effect of doping on the catalyst performance in ethylene polymerization was investigated. The results showed that the catalysts based on AlCl3‐doped MgCl2 support exhibited a slightly higher activity than did the MgCl2‐supported catalyst and the molecular weight distribution (MWD) of polyethylene (PE) markedly increased (from 10.8 to 47.9) with the increase of AlCl3 content in catalysts. The changes in catalyst's active center distribution were studied based on nonlinear fitting of the polymer GPC curves by multiple Flory functions. It was found that increase of types of active centers by introducing AlCl3 into the support should be responsible for the broadening of MWD of PE. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1768–1772, 2006 相似文献
998.
Xin Tong Chang Liu Hui‐Ming Cheng Haichao Zhao Feng Yang Xuequan Zhang 《应用聚合物科学杂志》2004,92(6):3697-3700
Single‐walled carbon nanotubes (SWNTs) were modified with polyethylene (PE) prepared by in situ Ziegler–Natta polymerization. Because of the catalyst pretreated on the surface of the SWNTs, the ethylene was expected to polymerize there. Scanning electron microscopy images and solubility measurements showed that the surface of the SWNTs was covered with a PE layer, and a crosslink may have formed between the SWNTs and PE. When the SWNTs covered with a PE layer were mixed with commercialized PE by melt blending, the resulting composite had better mechanical properties than the composite from the SWNTs without a PE layer. The yield strength, the tensile strength and modulus, the strain at break, and the fracture energy of the modified‐SWNT/PE composites were improved by 25, 15.2, 25.4, 21, and 38% in comparison with those of the raw‐SWNT/PE composites. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3697–3700, 2004 相似文献
999.
Polycaproamide (PCA) and cationic dyeable poly(ethylene terephthalate) (CDP) polymers were blended mechanically (in ratios of 75/25, 50/50, and 25/75) in a melt twin‐screw extruder to prepare three PCA/CDP polyblended materials. The blends of PCA and CDP were spun into fibers. The molar ratio of dimethyl 5‐sulfoisophthalate sodium salt for CDP was 2%. This study investigated the physical properties of PCA/CDP polyblended fibers with nuclear magnetic resonance, gel permeation chromatography, gas chromatography, potentiometer, differential scanning calorimetry (DSC), thermogravimetric analysis, scanning electron microscopy (SEM), extension stress–strain measurements, density gradient analysis, and rheometry. The experimental results of DSC proved that PCA and CDP formed an immiscible system. In an SEM image of a 50/50 PCA/CDP blend, the morphological aggregation of a larger size, from 3 to 5 μm in diameter, was observed. The rheological behavior of the PCA/CDP polyblended materials exhibited negative‐deviation blends, and the 50/50 blend of the PCA/CDP polyblended fibers showed a minimum tenacity value. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1710–1715, 2004 相似文献
1000.
Sun Ying-Chieh; Veenstra David L.; Kollman Peter A. 《Protein engineering, design & selection : PEDS》1996,9(3):273-281
Free energy calculations were carried out to determine the relativeunfolding free energy of the Ile96 wild type and Ala96 mutantbarnases. The total calculated free energies suggest that substitutionof Ile96 with Ala destabilizes barnase by 3.9 kcal/mol, whichis in good agreement with the independently determined experimentalvalues of 4.0 and 3.3 kcal/mol and a previous simulation. However,a decomposition of the free energy finds the dominant contributionsto this free energy arising from the noncovalent Interactionsbetween the perturbed group and distant residues of barnasein the sequence and water molecules and only a very small contributionfrom covalent interactions. This is in contrast to the previoussimulation, using the dual topology methodology, which produceda decomposition with an {small tilde}60% free energy contributionfrom changes in covalent interactions. The use of the singletopology employed in the present calculations and the dual topologyemployed in the previous study are analyzed in order to understandthe contrast between the present results and the results ofthe previous study. 相似文献