首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1899篇
  免费   99篇
  国内免费   7篇
电工技术   27篇
综合类   2篇
化学工业   412篇
金属工艺   70篇
机械仪表   137篇
建筑科学   31篇
矿业工程   4篇
能源动力   94篇
轻工业   131篇
水利工程   12篇
石油天然气   4篇
武器工业   1篇
无线电   291篇
一般工业技术   393篇
冶金工业   113篇
原子能技术   21篇
自动化技术   262篇
  2024年   2篇
  2023年   39篇
  2022年   38篇
  2021年   77篇
  2020年   58篇
  2019年   75篇
  2018年   62篇
  2017年   59篇
  2016年   80篇
  2015年   51篇
  2014年   115篇
  2013年   146篇
  2012年   136篇
  2011年   145篇
  2010年   96篇
  2009年   99篇
  2008年   99篇
  2007年   98篇
  2006年   79篇
  2005年   51篇
  2004年   50篇
  2003年   47篇
  2002年   21篇
  2001年   39篇
  2000年   23篇
  1999年   25篇
  1998年   43篇
  1997年   24篇
  1996年   19篇
  1995年   12篇
  1994年   9篇
  1993年   16篇
  1992年   12篇
  1991年   3篇
  1990年   6篇
  1989年   9篇
  1988年   4篇
  1987年   6篇
  1986年   3篇
  1985年   4篇
  1983年   3篇
  1982年   2篇
  1981年   2篇
  1977年   7篇
  1976年   2篇
  1975年   2篇
  1974年   2篇
  1973年   3篇
  1964年   1篇
  1941年   1篇
排序方式: 共有2005条查询结果,搜索用时 15 毫秒
101.
The thermoelectric properties at elevated temperature were investigated for n-type Bi2(Te,Se)3 which is obtained from ball milling processed powder with various milling times. Electrical properties such as electrical resistivity and Seebeck coefficient are clearly dependent on milling time, in which the carrier concentration is attributed to the change of the electrical properties. The concentrations of the defects are also varied with the ball milling time, which is the origin of the carrier concentration variation. Even though finer grain sizes are obtained after the long ball milling time, the temperature dependence of the thermal conductivity is not solely understood with the grain size, whereas the electrical contribution to the thermal conductivity should be also considered. The highest figure of merit value of ZT = 0.83 is achieved at373 K for the optimized samples, in which ball milling time is 10 h. The obtained ZT value is 48% improvement over that of the 0.5-h sample at 373 K.  相似文献   
102.
We present a new comprehensive scheme for generating grain boundary conformed, volumetric mesh elements from a three-dimensional voxellated polycrystalline microstructure. From the voxellated image of a polycrystalline microstructure obtained from the Monte Carlo Potts model in the context of isotropic normal grain growth simulation, its grain boundary network is approximated as a curvature-maintained conformal triangular surface mesh using a set of in-house codes. In order to improve the surface mesh quality and to adjust mesh resolution, various re-meshing techniques in a commercial software are applied to the approximated grain boundary mesh. It is found that the aspect ratio, the minimum angle and the Jacobian value of the re-meshed surface triangular mesh are successfully improved. Using such an enhanced surface mesh, conformal volumetric tetrahedral elements of the polycrystalline microstructure are created using a commercial software, again. The resultant mesh seamlessly retains the short- and long-range curvature of grain boundaries and junctions as well as the realistic morphology of the grains inside the polycrystal. It is noted that the proposed scheme is the first to successfully generate three-dimensional mesh elements for polycrystals with high enough quality to be used for the microstructure-based finite element analysis, while the realistic characteristics of grain boundaries and grains are maintained from the corresponding voxellated microstructure image.  相似文献   
103.
Oligomers consisting of alternating phenylethynyl and monosilyl or disilanyl moieties were synthesized in 15–64% precipitated yields by the reactions of dilithiated ethynylbenzene with dichlorodimethylsilane, dichlorodiphenylsilane, and 1,2-dichloro-1,1,2,2-tetramethyldisilane. Degrees of polymerization were fairly low due to chain termination reactions involving the deprotonation of ethynyl groups by aryllithium species. The presence of butyl chain ends was confirmed by 1H and 29Si NMR spectroscopy. 29Si NMR spectroscopy was utilized to illustrate the random pattern of connectivity along the oligomer backbones. The oligomers exhibit fluorescent behavior in solution.  相似文献   
104.
In order to investigate the effect of solution concentration on the surface properties of Y2O3 films dipcoated on a flexible metal substrate, films were coated on unpolished Hastelloy substrate followed by a heat treatment process to convert the film to amorphous Y2O3. Solutions of different molarities were used to deposit films with different thicknesses, and the surface smoothness and morphologies were investigated. The thickness of the film increased along with the molarity of the coating solution. The rate of improvement of the smoothness was fast when solutions of high molarity were used; this rate improvement can help reduce the number of coatings needed to reach the desired level of smoothness. It was found that multiple coated Y2O3 film has a smooth surface because the film covers up the irregular surface contours of the substrate.  相似文献   
105.
Hypertension induces renal fibrosis or tubular interstitial fibrosis, which eventually results in end-stage renal disease. Epithelial-to-mesenchymal transition (EMT) is one of the underlying mechanisms of renal fibrosis. Though previous studies showed that Ecklonia cava extracts (ECE) and dieckol (DK) had inhibitory action on angiotensin (Ang) I-converting enzyme, which converts Ang I to Ang II. It is known that Ang II is involved in renal fibrosis; however, it was not evaluated whether ECE or DK attenuated hypertensive nephropathy by decreasing EMT. In this study, the effect of ECE and DK on decreasing Ang II and its down signal pathway of angiotensin type 1 receptor (AT1R)/TGFβ/SMAD, which is related with the EMT and restoring renal function in spontaneously hypertensive rats (SHRs), was investigated. Either ECE or DK significantly decreased the serum level of Ang II in the SHRs. Moreover, the renal expression of AT1R/TGFβ/SMAD was decreased by the administration of either ECE or DK. The mesenchymal cell markers in the kidney of SHRs was significantly decreased by ECE or DK. The fibrotic tissue of the kidney of SHRs was also significantly decreased by ECE or DK. The ratio of urine albumin/creatinine of SHRs was significantly decreased by ECE or DK. Overall, the results of this study indicate that ECE and DK decreased the serum levels of Ang II and expression of AT1R/TGFβ/SMAD, and then decreased the EMT and renal fibrosis in SHRs. Furthermore, the decrease in EMT and renal fibrosis could lead to the restoration of renal function. It seems that ECE or DK could be beneficial for decreasing hypertensive nephropathy by decreasing EMT and renal fibrosis.  相似文献   
106.
Simulated microgravity (SMG) induced the changes in cell proliferation and cytoskeleton organization, which plays an important factor in various cellular processes. The inhibition in cell cycle progression has been considered to be one of the main causes of proliferation inhibition in cells under SMG, but their mechanisms are still not fully understood. This study aimed to evaluate the effects of SMG on the proliferative ability and cytoskeleton changes of Chang Liver Cells (CCL-13). CCL-13 cells were induced SMG by 3D clinostat for 72 h, while the control group were treated in normal gravity at the same time. The results showed that SMG reduced CCL-13 cell proliferation by an increase in the number of CCL-13 cells in G0/G1 phase. This cell cycle phase arrest of CCL-13 cells was due to a downregulation of cell cycle-related proteins, such as cyclin A1 and A2, cyclin D1, and cyclin-dependent kinase 6 (Cdk6). SMG-exposed CCL-13 cells also exhibited a downregulation of α-tubulin 3 and β-actin which induced the cytoskeleton reorganization. These results suggested that the inhibited proliferation of SMG-exposed CCL-13 cells could be associate with the attenuation of major cell cycle regulators and main cytoskeletal proteins.  相似文献   
107.
In this study a single-well, "push- pull" test method is adapted for determination of in situ denitrification rates in groundwater aquifers. The rates of stepwise reduction of nitrate to nitrite, nitrous oxide, and molecular nitrogen were determined by performing a series of push-pull tests. The method consists of the controlled injection of a prepared test solution ("push") into an aquifer followed by the extraction of the test solution/ground water mixture ("pull") from the same location. The injected test solution consists of ground water containing a nonreactive tracer and one or more biologically reactive solutes. Reaction rate coefficients are computed from the mass of reactant consumed and/or product formed. A single Transport Test, one Biostimulation Test, and four Activity Tests were conducted for this study. Transport tests are conducted to evaluate the mobility of solutes used in subsequent tests. These included bromide (a conservative tracer), fumarate (a carbon and/or source), and nitrate (an electron acceptor). Extraction phase breakthrough curves for all solutes were similar, indicating apparent conservative transport of the solutes prior to biostimulation. Biostimulation tests were conducted to stimulate the activity of indigenous heterotrophic denitrifying microorganisms and consisted of injection of site ground water containing fumarate and nitrate. Biostimulation was detected by the simultaneous production of carbon dioxide and nitrite after each injection. Activity tests were conducted to quantify rates of nitrate, nitrite, and nitrous oxide reduction. Estimated zero-order degradation rates decreased in the order nitrate > nitrite > nitrous oxide. The series of push-pull tests developed and field tested in this study should prove useful for conducting rapid, low-cost feasibility assessments for in situ denitrification in nitrate-contaminated aquifers.  相似文献   
108.
In this study, nano-crystalline LiCoO2 was coated onto the surface of Li1.05Ni0.35Co0.25Mn0.4O2 powders via sol–gel method. The influence of the coating on the electrochemical behavior of Li1.05Ni0.35Co0.25Mn0.4O2 is discussed. The surface morphology was characterized by transmission electron microscopy (TEM). Nano-crystallized LiCoO2 was clearly observed on the surfaces of Li1.05Ni0.35Co0.25Mn0.4O2. The phase and structural changes of the cathode materials before and after coating were revealed by X-ray diffraction spectroscopy (XRD). It was found that LiCoO2 coated Li1.05Ni0.35Co0.25Mn0.4O2 cathode material exhibited distinct surface morphology and lattice constants. Cyclic voltammetry (2.8–4.6 V versus Li/Li+) shows that the characteristic voltage transitions on cycling exhibited by the uncoated material are suppressed by the 7 wt.% LiCoO2 coating. This behavior implies that LiCoO2 inhibits structural change of Li1.05Ni0.35Co0.25Mn0.4O2 or reaction with the electrolyte on cycling. In addition, the LiCoO2 coating on Li1.05Ni0.35Co0.25Mn0.4O2 significantly improves the rate capability over the range 0.1–4.0C. Comparative data for the coated and uncoated materials are presented and discussed.  相似文献   
109.
A numerical method is presented for simulating film boiling on an immersed (or irregularly shaped) solid surface. The level set formulation for tracking the phase interfaces is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip condition at the fluid–solid interface. The boundary or matching conditions at the phase interfaces are accurately imposed by incorporating the ghost fluid approach based on a sharp-interface representation. The numerical method is tested through computations of bubble rise in a stationary liquid, single-phase fluid flow past a circular cylinder, and film boiling on a horizontal cylinder.  相似文献   
110.
In the present study, a numerical simulation of flame propagation and extinction in a micro combustor that is subject to excessive heat loss to the wall, particularly during flame propagation, is described. Heat loss to the wall was empirically modeled from measurement data on a similarly configured micro combustor. A PISO based numerical scheme was used for differencing the conservation equations. An H2-air reaction mechanism involving 16 species and 10 reaction steps was used to approximate the combustion process. A cylindrical computation domain was used to simulate the experiments. The combustor volume has a small height to radius ratio and an axial gradient of properties can be significant. In the present study, however, axial gradients were ignored, leaving radius as the only spatial coordinate. Instead of evaluating heat transfer from the temperature gradient near the wall surface, an empirical bulk heat transfer coefficient was used to approximate heat loss to the wall. A comparison of the computation and measurements showed a good agreement in flame propagation speed and cooling process, after the flame had been quenched by an artificially imposed extinction condition.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号